backthrusting in accretionary wedges backthrusting of the rear of the accretionary wedge arcward over the rocks of the forearc basin is a common aspect of accretionary tectonics an older assumption that backstops of accretionary wedges dip back toward the arc and that accreted material is emplaced below such backstops is contradicted by observations from many active forearcs that indicate 1 backthrusting is common 2 forearc basins are nearly ubiquitous associates of accretionary wedges and 3 forearc basement where imaged appears to diverge from the sedimentary package dipping under the wedge while the overlying sediments are often lifted up against it backthrusting may be favored where relief is high between the crest of the wedge and the surface of the forearc basin because the relief must be supported by shear stress along the backthrust we find ratios of lt lb averaging 2 64 0 8 where lt is the horizontal distance from the crest of the wedge to the front of the toe and lb is the distance from the crest of the wedge to the surface projection of the backthrust this ratio is predicted to increase with total convergence and with sedimentation because the toe undergoes higher deformation rates than the backthrust and sedimentation in the trench favors outbuilding of the toe whereas sedimentation in the forearc basin tends to cancel the gravitational component of the driving stress for the backthrust

fifteen day observation of mesospheric and lower thermospheric motions with the aid of the arecibo uhf radar an attempt was made to have a continuous observation of the zonal wind component in the tropical mesosphere and lower thermosphere for 15 days of august 1 15 1980 with the aid of the uhf radar at arecibo 18 n by using the wind data together with additional data obtained from meteorological rockets meteor radar and tiros n satellite analyses were made of the time mean zonal wind profile tides long period waves and short period gravity type waves respectively there appears a remarkable day to day variation of the zonal wind throughout the region between 70 and 95 km with a characteristic time scale of about 5 days indicating the passage of traveling planetary scale waves in the summertime tropical middle atmosphere it was also found that the short period oscillation with a time scale of a few 10 min appears intermittently in association with the strong vertical shear of the background flow this oscillation is considered to be the manifestation of the kelvin helmholtz type instability in the lower thermosphere

maps of polar mesospheric clouds 1 a middle ultraviolet imager 235 263 nm on the midcourse space experiment msx has obtained the first large scale two dimensional maps of polar mesospheric clouds pmcs the lower parts of individual images which lie below the ozone horizon are mapped onto an ellipsoidal shell at 83 km altitude and then combined to establish the transpolar pmc field in two spatial dimensions across the entire polar region at all latitudes where they appear the pmcs clearly evidence a patchy structure as opposed to a uniform layer among the interesting features of these cloud patches are zonal alignments arcs and repetitive structures characteristic of waves whether random or repetitive the cloud structures exhibit scales ranging from several hundred kilometers down to tens of kilometers

lithospheric flexure under a critically tapered mountain belt a new technique to study the evolution of the tertiary taiwan orogeny abstract a joint analysis of both the lithospheric flexure of an underlying plate and mountain building mechanics is developed based on the critical wedge theory and the flexure response of an elastic plate over an inviscid asthenosphere according to the analysis the amount of deflection beneath a critically tapered mountain belt strongly depends upon the internal and basal friction coefficients of that belt and the flexural rigidity of the underlying plate rather than the horizontal stress induced by plate convergence an increase in the internal friction of a mountain belt or a decrease in the friction along its basal decollement tends to enlarge the deflection of the underlying plate the tendency toward an increased deflection of the underlying plate due to a decrease in flexural rigidity is suppressed by the development of a more narrowly tapered mountain belt the newly developed technique presented here is used to analyze the evolution of the tertiary taiwan orogeny and shows that the optimal flexural rigidity of the eurasian plate in taiwan is about 3 10 22 nm it also demonstrates that the collision between the eurasian plate and the luzon arc might have initiated 7 myr ago a finding that would account for the burial history of the foreland basin in south central taiwan

great earthquake surface ruptures along backthrust of the janauri anticline nw himalaya abstract results of a paleoseismic trenching investigation on a backthrust in the northern margin of janauri hill at mehandpur 31 18 11 37 n 76 18 31 47 e sub himalayan range of the himachal himalaya are presented the active backthrust revealed the last event took place after 0 8 0 03 ka ago i e post ce 1200 the age of the last earthquake with a minimum fault slip 1 m along f1 fault and presence of single colluvium on the back thrust nearly matches with an earthquake event previously documented on the bhatpur forethrust of the himalayan frontal thrust system uplifted and truncated fluvial terraces due to displacement on the backthrust are preserved along several north flowing river valley sections in the northern limb of janauri anticline the long term vertical uplift rate calculated through dividing difference between current elevation and elevation of current river grade is 1 08 0 08 mm yr field evidence shows fore and backthrusts are active and they form simultaneously but displacements are episodic suggesting the backthrust develops either due to locking of forethrust or to accommodate a large amount of fault slip due to magnitude of earthquake event along the forethrust further the backthrust may be used as an indirect method of inferring the age of the last event that took place along the forethrust

mountain building in taiwan a thermokinematic model the taiwan mountain belt is classically viewed as a case example of a critical wedge growing essentially by frontal accretion and therefore submitted to distributed shortening however a number of observations call for a significant contribution of underplating to the growth of the orogenic wedge we propose here a new thermokinematic model of the taiwan mountain belt reconciling existing kinematic thermometric and thermochronological constraints in this model shortening across the orogen is absorbed by slip on the most frontal faults of the foothills crustal thickening and exhumation are sustained by underplating beneath the easternmost portion of the wedge tananao complex tc where the uplift rate is estimated to 6 3 mm a 1 and beneath the westernmost internal region of the orogen hsueshan range units hr where the uplift rate is estimated to 4 2 mm a 1 our model suggests that the tc units experienced a synchronous evolution along strike despite the southward propagation of the collision it also indicates that they have reached a steady state in terms of cooling ages but not in terms of peak metamorphic temperatures exhumation of the hr units increases northward but has not yet reached an exhumational steady state presently frontal accretion accounts for less than 10 of the incoming flux of material into the orogen although there is indication that it was contributing substantially more 80 before 4 ma the incoming flux of material accreted beneath the tc significantly increased 1 5 ma ago our results also suggest that the flux of material accreted to the orogen corresponds to the top 7 km of the upper crust of the underthrust chinese margin this indicates that a significant amount 76 of the underthrust material has been subducted into the mantle probably because of the increase in density associated with metamorphism we also show that the density distribution resulting from metamorphism within the orogenic wedge explains well the topography and the gravity field by combining available geological data on the thermal and kinematic evolution of the wedge our study sheds new light onto mountain building processes in taiwan and allows for reappraising the initial structural architecture of the passive margin

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

a viscoelastic earthquake cycle model for taiwan 1 while recent geophysical measurements including global positioning system gps surveys have clarified the mechanisms of active mountain building in the taiwan arc continent collision the geometry of faults beneath the island remains unclear in this paper we use gps data collected before the 1999 chi chi earthquake to image the geometry of active faults under taiwan and estimate slip rates on these faults we invert the data using kinematic and mechanical earthquake cycle models consisting of faulting in an elastic lithosphere overlying a viscoelastic asthenosphere in the kinematic model uniform slip is prescribed on the faults at a constant rate while in the mechanical model the faults slip in response to shear stresses in the lithosphere these models explain the essential features of both the horizontal and vertical gps velocity fields we find a 27 39 km thick elastic lithosphere overlying a viscoelastic asthenosphere with maxwell relaxation time of 15 80 years or viscosity of 0 5 4 1019 pa s the models produce a combined slip rate of 30 50 mm yr on the frontal thrusts in western taiwan and 60 80 mm yr of slip on the longitudinal valley fault inversion of the data using both models indicates a decollement dipping 10 under western taiwan and increasing in dip by no more than 10 under the central ranges and eastern taiwan

seismogenic structure in a tectonic suture zone with new constraints from 2006 mw6 1 taitung earthquake 1 a mw 6 1 earthquake occurred on april 1st 2006 near taitung eastern taiwan it produced significant coseismic ground displacements and a large number of aftershocks in the ensuing month this event provides an opportunity to diagnose the seismogenic structure in the southern longitudinal valley lv of eastern taiwan long viewed as one of the collision sutures between the philippine sea and the eurasian plates with precisely relocated main and aftershock hypocenters focal mechanisms for m 3 8 events and coseismic ground displacements from strong motion records we determine a main shock dislocation model our results indicate that the main shock occurred on a high angle fault azimuth 198 dip 77 the model comprises a fault with two segments the main shock and a large number of aftershocks are associated with the northern segment that exhibited predominantly left lateral strike slip motion in agreement with p wave first motions and waveform usgs solutions the southern segment exhibits a slightly larger thrust component in agreement with cmt solutions tectonically this event highlights a nne trending fault on the west side of the lv which is predominantly strike slip the aftershocks clustered to the east of the main shock which exhibit mainly thrust mechanisms indicate that shortening is still acting on the sedimentary materials deposited between the coastal and central ranges prior to collision as a result the southern lv is undergoing slip partitioning along different faults which has never been specified before

anomalous behaviour of precursor resistivity in shillong area ne india summary daily measurement of apparent resistivity from 1984 may to 1985 october in shillong plateau india has been carried out for an earthquake precursor study the resistivity changes which occur 7 to 10 days before earthquakes do not necessarily satisfy the dilatancy hypothesis which of late has been subjected to criticisms of not being a generalized phenomenological explanation of precursor resistivity variation prior to earthquakes resistivity may increase or decrease or may not even show any change depending on the orientation of measuring electrodes the elastic nature of the geological formation where observations are made and the direction of tectonic stress

crustal deformation in taiwan results from finite source inversions of six mw 5 8 chi chi aftershocks 1 we inverted strong motion data for the finite source parameters of six large aftershocks of the 1999 chi chi taiwan earthquake to investigate seismogenic structure in taiwan for each event we derived a preferred model by testing different focal mechanisms hypocenters rupture velocities and dislocation risetimes as well as different combinations of stations in more than 1000 inversions we documented how the fits between the waveforms and the corresponding synthetics deteriorated as the hypocenter and focal mechanism deviate from those of the preferred model if the deviation in focal mechanisms and hypocenters is less than 20 and 5 km respectively we generally recovered 80 of the preferred model s synthetic waveform fit unlike the dislocation risetime the rupture velocity used in the inversion had a strong influence on the waveform fits in this study we also used the slip models to study fault geometry two of the aftershocks ruptured on the southern extension of the main shock slip area one strike slip aftershock nucleated within the basement but ruptured mainly within the overlying sedimentary strata suggesting that seismogenic deformation in the basement can influence shallow structures p axes of the derived models have azimuths consistent with current plate motion finally gps displacement derived from the six slip models can explain 80 of the postseismic deformation observed in the aftershock regions indicating that studies of postseismic deformation must take into account the cumulative effects of large shallow aftershocks

inversion of the paleogene chinese continental margin and thick skinned deformation in the western foreland of taiwan abstract new structural data available seismicity data together with mechanical constraints on the eurasian continental lithosphere and reconstruction of paleostress trajectories are combined in order to re assess and discuss the dominant deformation mode thin skinned vs thick skinned in the western fold and thrust belt of the active taiwan collision zone serial balanced cross sections and computed paleostress tensors suggest that structural styles and stress trajectories at the front of the taiwan mountain belt may vary rapidly along strike depending on the presence of pre orogenic paleogene troughs in the chinese continental margin taihsi and tainan basins that are favourably oriented to be reactivated and inverted in localities of the western foreland where basin inversion and thick skinned basement involved shortening predominate the crustal seismic activity is important and characterized by strike slip faulting in these domains of the fold thrust belt the stress deviations with respect to the regional transport direction are also important by contrast in domains of significant syn orogenic subsidence a thin skinned style of deformation may be prominent due to the lack of available pre existing features the seismic activity is limited to few major faulted boundaries such as the active chelungpu sani thrust and the stress deviations are limited the timing of deformation at the belt front seems to be independent of the structural styles so that frontal folds are active since probably 0 5 ma however in inner parts of the western foothills of taiwan where superimposed thick skinned and thin skinned deformation are required out of sequence thrusting occurs to maintain the topographic profile finally we suggest that kinematics and structural styles of the western fold thrust belt are controlled by the mechanics of the eurasian continental margin in agreement with a recent study at the scale of the orogen the limited shortening amounts across the wf and in the central range as well as the absence of continental hp rocks better support a thick skinned collision model for taiwan

magnetic properties of anorthosites a forgotten source for planetary magnetic anomalies 1 anorthosites igneous rocks very rich in plagioclase rarely considered to be strongly magnetic are common on earth and the moon and inferred to be on other planets magnetic properties of anorthosites could be important in investigating associated mineral deposits and in studying magnetic anomalies especially on mars here we investigate three late proterozoic anorthosites in rogaland norway for magnetic and petrographic properties two of the anorthosites have large natural remanent magnetization nrm with intensities comparable to tertiary basalts susceptibility nrm and hysteresis properties provide information about the magnetic minerals present and their response to inducing fields microscopic observations show ubiquitous hemo ilmenite in the anorthosites whereas magnetite is common in the haland helleren but rare in the ana sira and egersund ogna bodies this study illustrates that anorthosites can be important sources of magnetic anomalies and can retain a remanent field over geologic time it also supports the recently described property of lamellar magnetization

continuous monitoring of an active fault in a plate suture zone a creepmeter study of the chihshang fault eastern taiwan abstract data from continuously monitored creepmeters across the active chihshang fault in eastern taiwan are presented the chihshang fault is an active segment of the longitudinal valley fault the main suture between the converging philippine and eurasian plates in taiwan since the 1951 earthquake mw 7 0 no earthquake larger than magnitude 6 0 occurred in the chihshang area at least during the last 20 years the chihshang fault underwent a steady creep movement resulting in numerous fractures at the surface five creepmeters were installed in 1998 at two sites tapo and chinyuan within the chihshang active fault zone one year results from august 1998 to july 1999 show a horizontal shortening of 19 4 0 3 mm and 17 3 0 7 mm at tapo and chinyuan respectively these annual shortening rates are in a good agreement with other estimates of strain rate independently obtained from geodetic measurements and geological site investigation the creepmeter measurements were made on a daily basis providing accurate information on the previously unknown evolution of creep during the year the records of fault creep at the tapo site thus revealed close seasonal correlation with average rainfall the period of high creep rate coincides with the wet season whereas that of low creep rate coincides with the dry season also in comparison with the tapo site the creep behaviour as a function of time is complex at the chinyuan site possible factors of irregularity are under investigation thermal effect acting on the concrete basement of the creepmeters earth tide effect water table variations in a nearby rice field and rainfall the comparison between gps measurements across the longitudinal valley 31 mm year of horizontal displacement and the creepmeter measurement across the chihshang fault zone 17 19 mm year of horizontal displacement suggests that there exists other shortening deformation across the active fault zone in addition to those we have measured from the creepmeters

quasi periodic radar echoes from midlatitude sporadic e and role of the 5 day planetary wave using measurements of magnetic aspect sensitive radar echoes from midlatitude sporadic e collected over a two month period from tanegashima japan we show that while their occurrence duration from night to night did not exhibit any systematic variation that of the so called quasi periodic qp echoes varied sinusoidally with a period of 5 days we have interpreted this behavior in terms of effects produced by a planetary wave and identified its presence through neutral wind measurements made with a partial reflection drift radar located nearby at yamagawa we propose that the occurrence of qp echoes is affected both by a contribution of the wind to the dynamo electric field and by the direction of the neutral wind we argue that because the wind vector of the planetary wave is elliptically polarized at midlatitudes a preferred wind direction conducive to the generation of qp echoes occurs once every 5 days on the other hand this wave is linearly polarized and directed zonally over the geographic equator the fact that qp echoes are most fully developed at midlatitudes and less so at lower latitudes suggests that zonal flow is not particularly favorable for qp echo production

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

long term observations of the arctic mesosphere with the mst radar at poker flat alaska in this report we summarize the results of 22 months of observations of arctic mesosphere echoes with the 50 mhz radar at poker flat alaska operation with a partially complete system began in february 1979 and has continued on a nearly continuous basis to the present time the altitude range of the mesospheric echoes obtained during this period shows a pronounced seasonal variation during summer months the mesospheric echoes are relatively continuous in time and extend from about 80 to 100 km with an unexpectedly strong peak in signal to noise ratio at about 85 km in contrast in nonsummer months mesospheric echoes are less intense and occur at lower altitudes in the range from 55 to 80 km these lower altitude nonsummer echoes are observed primarily during daytime hours when energetic auroral particle precipitation as determined by the poker flat 30 mhz riometer is present

conjugate thrust faulting associated with the 1999 chi chi taiwan earthquake sequence 1 the geometrical structure of the responsible faults of the 20 september 1999 chi chi taiwan earthquake ml 7 3 mw 7 6 and its aftershocks can be clearly depicted by well located hypocenters and focal mechanisms of large aftershocks the mainshock and two large aftershocks with ml 6 8 were characterized by thrust faulting along a n s striking fault plane dipping to the east the underground structure of the chelungpu fault which is probably merging with the decollement beneath the western foothills can be clearly associated with the seismicity pattern and the focal mechanisms of the three largest events a group of deeper aftershocks including two moderate events ml 6 3 and 6 0 respectively were located to the southeast of the mainshock along a fault plane dipping steeply to the west down to a depth of about 30 km our results suggest that the spatial pattern of the aftershocks in the southern part of the source area can be interpreted by a conjugate fault system this conjugate fault system is comprised of the gently east dipping chelungpu fault and a steeply west dipping deeper fault zone

five day mesospheric waves observed in solar mesosphere explorer ozone solar mesosphere explorer sme ozone data were analyzed for short term periodic oscillations in the altitude region from 50 to 90 km a 5 day oscillation was found to be a dominant feature at high southern latitudes near the december solstice in the altitude range from 50 to 80 km the wave feature appears to travel westward with a speed consistent with that of a wave number one planetary scale oscillation circling the globe in 5 days the amplitude also appears to be consistent with the temperature oscillations predicted by geisler and dickinson 1976 this oscillation was not found to be a dominant feature in sme ozone during northern hemisphere summer

synorogenic extension and exhumation of the taiwan hinterland structural data from two transects across the active taiwan arc continent collision indicate that normal faulting is an important mechanism in the exhumation of the hinterland the normal faulting extends to at least the brittle ductile transition records extension at a high angle to the topographic grain of the orogenic belt and is responsible for the regional scale rotation of ductile fabrics these results are supported by a recent gps survey of southern taiwan that reveals extension in the hinterland contemporaneous with shortening in the foreland and collided arc

variability of repeating earthquake behavior along the longitudinal valley fault zone of eastern taiwan 1 two major clusters of repeating earthquake sequences res are found on two fault patches near the longitudinal valley fault lvf in eastern taiwan the chihshang segment to the south and the hualien segment to the north the 25 m 2 3 4 6 res on the hualien segment are widely distributed in a nne direction over a distance of 45 km whereas on the chihshang segment the 30 m 2 2 3 4 res have a shorter along strike extent of about 15 km the longer spatial extent of res on the hualien segment may indicate a wider distributed creeping zone at depth m 3 8 res are observed on the hualien segment and are characterized by quasiperiodic recurrence statistical analysis of the size and number of earthquakes adjacent to the res indicates that the smaller number of large earthquakes in the vicinity may explain the quasiperiodic recurrence of res we argue that on the hualien segment m 4 or larger earthquakes are isolated enough in space to facilitate the occurrence of repeated ruptures

on steady states in mountain belts the dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state this concept is often invoked but the nature of the steady state is commonly not specified we identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical model results and natural examples these types are 1 flux steady state 2 topographic steady state 3 thermal steady state and 4 exhumational steady state they refer to the erosional flux the topography the subsurface temperature field and the spatial pattern of cooling ages respectively models suggest that the topography will reach a steady mean form at the scale of an orogenic belt but perfect topographic steady state is unlikely to be achieved at shorter length scales thermal steady state is a precondition for exhumational steady state and in the case of temperature dependent deformation topographic steady state exhumational steady state is characterized by reset age zones spatially nested according to closure temperature as illustrated in natural systems from new zealand the cascadia accretionary margin and taiwan

a review of gravity wave saturation processes effects and variability in the middle atmosphere this paper provides a review of our current understanding of the processes responsible for gravity wave saturation as well as the principal effects and variability of saturation in the lower and middle atmosphere we discuss the theoretical and observational evidence for linear and nonlinear saturation processes and examine the consequences of saturation for wave amplitude limits momentum and energy fluxes the diffusion of heat and constituents and the establishment of a near universal vertical wavenumber spectrum recent studies of gravity wave variability are reviewed and are seen to provide insights into the significant causes of wave variability throughout the atmosphere

determining fault geometry from the distribution of coseismic fault slip related to the 2006 taitung earthquake eastern taiwan on 1 april 2006 the taitung earthquake m w 6 1 occurred in taiwan at the boundary between the philippine sea and eurasian plates where high convergence rates contributed to the development of plio pleistocene orogeny in the region from the joint inversion of seismic and geodetic data we identified the event s fault geometry and reconstructed the distribution of coseismic fault slip we modeled fault geometries with increasing complexity and selected the model that best reproduced all datasets simultaneously even though the earthquake magnitude was moderate rupturing occurred in two steps the initial rupture was generated on a listric north south trending fault for which dip decreases with increasing depth and was immediately followed by movement along a perpendicular structure that cross cuts the main fault at 5 km south of the earthquake hypocenter the average slip along the rupture was 30 cm with a maximum of 87 cm oblique reverse fault movement was characterized by a predominant left lateral component the amount of slip is well constrained for offsets of more than 5 cm with an associated uncertainty of 32 for slip amounts greater than 5 cm uncertainties on rake and rupture time are 11 and 0 54 s respectively the rupture propagated from the hypocenter bilaterally moving slightly faster toward the south 2 5 0 4 km s than to the north 1 7 0 1 km s to the south the rupture was rapidly transmitted upward at the junction with the cross cutting east west segment whereas in the north the rupture remained confined to the lower segment of the main fault from global positioning systems gps and seismic data time window 1 min we infer that the cross cutting segment was activated following coseismic rupture on the main north south fault yet close enough in time to be associated with coseismic movement acquired by gps daily solutions

mean zonal acceleration and heating of the 70 to 100 km region the dynamical interactions which occur in the atmospheric region around the mesopause 90 km determine the boundary characteristics for the thermospheric region above a significant data base of radar wind measurements exists which characterizes the global circulation at altitudes between 70 and 110 km in the present work using an empirical model of eulerian mean meridional motions based on monthly climatological winds from these radar data the net vertical motions in this atmospheric regime are derived from the continuity equation assuming empirical prescriptions of the mean density and temperature fields mean heat flux divergences and momentum flux divergences are estimated which exhibit very specific characteristics in the height versus latitude domain for winter summer and equinox conditions in both hemispheres a numerical circulation model including gravity wave mean flow and tide mean flow interactions is utilized to examine possible origins of these heat and acceleration sources at low latitudes 30 it is evident that atmospheric tides represent the primary wave source contribution to zonal mean acceleration and heating of this region of the atmosphere similarly at middle and high latitudes 30 below about 90 km dissipation of vertically propagating gravity waves appears to provide the dominant momentum source for the mean zonal circulation however above approximately 90 km and between about 40 and 70 latitude very significant regions of mean heating and acceleration exist which are not accounted for by the effects of vertically propagating gravity waves and tides the possible origins of these effects are examined we suggest that the two most likely candidates to explain these observed features are 1 obliquely propagating gravity waves and or 2 planetary scale waves

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

seasonal variations of the gravity wave momentum flux in the antarctic mesosphere and lower thermosphere airglow imager and dynasonde imaging doppler interferometer idi radar wind measurements at halley station antarctica 75 6degreess 26 6degreesw have been used to estimate the seasonal variation of the vertical fluxes of horizontal momentum carried by high frequency atmospheric gravity waves the cross correlation coefficients between the vertical and horizontal wind perturbations were calculated from sodium na airglow imager data collected during the austral winter seasons of 2000 and 2001 these were combined with wind velocity variances from coincident radar measurements to estimate the daily averaged upper limit of the vertical flux of horizontal momentum due to gravity waves the resulting momentum flux at the na airglow altitudes while displaying a large day to day variability showed a marked rotation from the northwest to the southeast throughout the winter season calculations show that this rotation is consistent with seasonal changes in the wind field filtering of gravity waves below the na airglow region the calculations also indicate that while the magnitude of the meridional wind is small this filtering leads to the observed seasonal changes in the meridional momentum flux

active back thrust in the eastern taiwan suture revealed by the 2013 rueisuei earthquake evidence for a doubly vergent orogenic wedge rapid exhumation of 3 10 mm yr of the taiwan metamorphic range is often explained as the unroofing of the retrowedge of a doubly vergent mountain belt yet to date the central range fault forming the boundary of the retrowedge has displayed no definitive evidence for recent seismic activity and no unambiguous geomorphic expression over much of the fault the 2013 m6 4 rueisuei reverse faulting earthquake nucleated at the eastern boundary of the retrowedge and appears to illuminate the west dipping central range fault we estimate the fault geometry and coseismic slip distribution using a uniform stress drop slip inversion and surface displacements derived from gps and strong motion data we identify a 42 dipping blind reverse fault consistent with the previously proposed buried central range fault beneath the highly active longitudinal valley fault this earthquake may be the first indication that rapid exhumation and uplift occur along a distinct fault structure bounding the eastern margin of the taiwan retrowedge

characteristics of instabilities in the mesopause region over maui hawaii 1 characteristics of convective and dynamical instabilities in the mesopause region between 85 and 100 km over maui hawaii 20 7 n 156 3 w are investigated using 19 nights 133 hours of high resolution wind and temperature data obtained by the university of illinois na wind temperature lidar during the maui mesosphere and lower thermosphere maui malt campaigns the mean probabilities of convective and dynamical instabilities are observed to be 3 and 10 respectively but there is considerable night to night variation at any given time the probability that an unstable condition is found at some altitudes in the 85 100 km range is 90 the maui malt data exhibit a distinct trend for n2 to increase with wind shear and vice versa this correlation has important implications in the understanding of the development of instabilities the night of 11 april 2002 is studied in detail in order to investigate the spatial and temporal structures of n2 wind shear and convective and dynamical instabilities a close linkage between instability and the mesosphere inversion layers mils is identified most of the convectively and dynamically unstable regions are located above the mils with a tendency for dynamical instability to develop below convective instability it is found that the vertical variations of n2 are often correlated with those of wind shear but with a phase shift such that the maxima and minima of n2 are located 0 5 1 km below those of wind shear because of this shift dynamical instability tends to develop in the region above the maximum wind shear where relatively small n2 is observed to be associated with large wind shear we also found that the wind shear is dominated by the contribution of the meridional wind especially when the wind shear is strong possible mechanisms for the observed features are discussed

uplift shortening and steady state topography in active mountain belts we present a tectonic surface process model used to investigate the role of horizontal shortening in convergent orogens and the effects on steady state topography the tectonic model consists of a specified velocity field for the earth9s surface and includes a constant uplift rate and a constant horizontal strain rate which varies to reflect the relative importance of frontal accretion and underplating in an orogenic wedge the surface process model includes incision of a network of rivers formed by collection of applied precipitation and diffusive hillslope mass transfer three non dimensional parameters describe this model a ratio of the maximum horizontal velocity to the vertical velocity a peclet number expressing the efficiency of the hillslope diffusion relative to the uplift rate and a fluvial erosion number reflecting the fluvial incision efficiency relative to the uplift rate a series of models are presented demonstrating the resultant steady state landforms parameterized by these three numbers a finite velocity ratio results in an asymmetric form to the model mountain range although the magnitude of the asymmetry also depends on the peclet number topographic steady state is achieved faster for models with no horizontal component to the velocity field with finite horizontal velocity topographic steady state is achieved only at the scale of the entire mountain range even the first order drainage basins are unstable with time in the presence of horizontal shortening we compare our model results to topographic profiles from active mountain ranges in taiwan new zealand and the olympic mountains of washington state all these examples exhibit asymmetric topographic form with the asymmetry consistent with the polarity of subduction suggesting that horizontal tectonic motion is affecting the macro geomorphic form of these ranges

mesospheric winds at low and mid latitudes in the southern hemisphere mesospheric wind measurements were made by using the partial reflection technique at a tropical location townsville 19 s and at a mid latitude location adelaide 35 s in june and november 1978 it is shown that the prevailing winds were typical of expected winter and summer type circulations in june and november respectively but there was a large difference in the amplitudes and structure of the diurnal tide between the two sets of observations in november the tide was stable with a large amplitude and it appeared to break at a height of 86 km the propagating s1 1 mode dominated in june however the tide was weak with large day to day variability and the evanescent s1 2 mode was dominant the semi diurnal tide amplitudes were relatively small but internal gravity wave amplitudes were significant spectral analyses showed that in the 2 to 8 hour period band the wave energy decreased as f k with k in the range 1 1 5 and it is also shown that the waves were losing energy as a function of height the inferred energy loss rates were 0 05 w kg 1 and a turbulent viscosity of about 500 m s 1 was estimated near the mesopause at both sites and in both seasons

seismogenic strain across the transition from fore arc slivering to collision in southern taiwan tectonic slivers have profound implications on the mechanical behavior of fore arcs and subduction zones southern taiwan is characterized by precollision fore arc slivering that likely exerts important controls on the evolution of collision inversion of focal mechanism solutions for seismogenic strain reveals spatially partitioned plate motion south of taiwan with dip slip faulting nearer the manila trench and strike slip faulting nearer the luzon arc to the north these kinematics are less clearly segregated and both types of faults appear to occur in the same volume of crust further north deformation is dominated by oblique slip on potentially reactivated steep reverse faults in the collision zone existing work on the active fault zones in taiwan suggests that the subduction to collision transition is marked by a releasing step between the strike slip fault that bounds the fore arc sliver and the oblique thrusts of the collision zone this configuration may help to explain the lack of fore arc basement and cover exposed in the suture zone marked by the longitudinal valley

inferred fault geometry and slip distribution of the 2010 jiashian taiwan earthquake is consistent with a thick skinned deformation model abstract we invert measurements of coseismic displacements from 139 continuously recorded gps sites from the 2010 jiashian taiwan earthquake to solve for fault geometry and slip distribution using an elastic uniform stress drop inversion the earthquake occurred at a depth of 23 km in an area between the western foothills fold and thrust belt and the crystalline high mountains of the central range providing an opportunity to examine the deep fault structure under taiwan the inferred rupture plane is oblique to the prominent orientation of thrust faults and parallel to several previously recognized nw striking transfer zones that appear to connect stepping thrusts we find that a fault striking 318 344 with dip of 26 41 fits the observations well with oblique reverse sinistral slip under a low stress drop of about 0 5 mpa the derived geodetic moment of 2 92 10 18 n m is equivalent to a m w 6 24 earthquake coseismic slip is largely concentrated within a circular patch with a 10 km radius at the depth between 10 and 24 km and maximum slip of 190 mm we suggest this earthquake ruptured the nw striking chishan transfer fault zone which we interpret as a listric ne dipping lateral ramp with oblique slip connecting stepping thrust faults ramps the inferred slip on the lateral ramp is considerably deeper than the 7 15 km deep detachment identified in previous studies of western taiwan we infer an active basal detachment under western taiwan at a depth of at least 20 23 km based on these inversion results the earthquake may have nucleated at the base of the lateral ramp near the intersection with the basal detachment coulomb stress change calculations suggest that this earthquake moved several ne striking active thrust faults in western taiwan nearer to failure

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

repeating earthquakes and seismic potential along the northern longitudinal valley fault of eastern taiwan 1 the northern longitudinal valley fault in eastern taiwan creeps at the surface with a small rate of 1 cm yr but slips in large earthquakes to improve seismic hazard assessment it is important to comprehend the slip deficit rate distribution at depth we discovered 25 ml 2 1 4 6 repeating earthquakes in this area and inverted gps measurements for producing an image of the along strike spatial distribution of deep fault slip rates the repeating events are located at the depths of 10 22 km with 24 9 77 5 mm yr slip rates which are comparable with the gps derived slip rates of 47 5 5 8 mm yr at similar depth ranges based on distribution of gps derived slip deficits since 1951 the northern longitudinal valley fault has become capable of releasing stored strain in a future mw 7 3 earthquake

kinematics of arc continent collision in the eastern central range of taiwan inferred from geomorphic analysis active arc continent collision in taiwan is an example of a typical collisional orogen with a deformed continental margin foothills belt hsueshan range and central range separated from an accreted island arc coastal range by a collapsed forearc basin longitudinal valley the geomorphology of the eastern central range reflects spatial and temporal variations in uplift rate and pattern along the orogen during late quaternary time digitized drainage basin perimeters stream channels and the mountain piedmont junction of a 200 km long segment of the eastern central range provide data for calculating quantitative geomorphic parameters statistical analyses of these parameters define four domains of distinct landscape character the northern domain extends northward along the coast from the northern end of the longitudinal valley lv and has landscape characteristics suggesting uplift near the mountain front the central two domains are adjacent to the lv and show evidence of uplift concentrated in the interior of the range away from the front the boundary between the two central domains occurs near the middle of the lv the southern domain extends southward along the coast from the southern end of the lv and has characteristics suggesting a transient response to increased uplift rate analysis of fluvial terrace profiles in the central range adjacent to the southern half of the lv indicates a change in uplift pattern through time with migration of maximum uplift rate from the mountain front to the interior of the range this change probably occurred just prior to late pleistocene time

magnetotelluric imaging beneath the taiwan orogen an arc continent collision 1 the taiwan orogen has formed since the late miocene by oblique collision between the luzon volcanic arc on the philippine sea plate and the eurasian continental margin this oblique collision has produced an orogen that decreases in age from north to south and permits study of the temporal evolution of an arc continent collision these factors make taiwan a favorable location to study the process of arc continent collision the first long period magnetotelluric mt measurements were recorded in taiwan as part of the taiwan integrated geodynamics research taiger project in 2006 7 measurements were made at 82 sites on three transects across south central and north taiwan that span the breadth of the orogen and cross all major tectonic boundaries robust remote reference processing of the mt time series data resulted in high quality soundings that were modeled in both 2 and 3 dimensions these mt models support predictions of lithospheric deformation i e thick skinned tectonics beneath the central ranges in south and central taiwan but are inconsistent with predictions of orogen scale thin skinned models the mt resistivity model for northern taiwan is consistent with dewatering of the subducting philippine slab and with deformation described by the subducting indenter tectonic model modeling the taiger mt data has definitively shown that conductive and seismically active crustal structures exist to 30 km beneath the orogen these conductive regions interpreted as interconnected fluid map pervasive zones of collisional deformation that are lithospheric in scale

active detachment of taiwan illuminated by small earthquakes and its control of first order topography we use 110 000 small earthquakes to locate and map active faults in three dimensions within the taiwan arc continent collision the structure is dominated by a nearly horizontal band of small earthquakes at 10 km depth that is interpreted to seismically illuminate the main detachment zone of the mountain belt with other illuminated fault zones abutting the detachment zone the zone steepens below eastern taiwan to 30 90 and reaches depths of 30 60 km the three dimensional shape of the detachment zone in relation to topography allows a new test of critical taper wedge mechanics and suggests that the reversal of topographic slope across taiwan is controlled by the shape of the detachment

insights into active tectonics of eastern taiwan from analyses of geodetic and geologic data 1 about 50 mm yr of convergence between the philippine sea and eurasian plates is absorbed in eastern taiwan and it remains unclear how the convergence is partitioned among active faults the longitudinal valley fault lvf the most seismically active fault in eastern taiwan creeps at the surface in the south and not in the north however it is unclear how much of the fault is locked or creeping at depth to address these problems we model holocene and interseismic deformation of elastic lithospheric blocks moving over a viscoelastic asthenosphere in eastern taiwan through a fully probabilistic scheme we invert gps interferometric synthetic aperture radar creepmeter and holocene marine terrace data for block motions fault slip rates and distribution of interseismic creep the data are explained with four blocks separated by three faults central range fault lvf and an offshore fault the model explains the essential features of interseismic and holocene deformation we find that 35 55 mm yr of slip on the offshore fault is necessary to fit marine terrace uplift rates which is a larger fraction of the plate convergence than previously recognized the lvf has a holocene slip rate of 20 30 mm yr with approximately equal magnitudes of reverse slip and left lateral strike slip components only about half of the surface area of the longitudinal valley fault appears to be locked the southern segment of the lvf creeps at a rate of 5 28 mm yr down to a depth of 15 20 km while the northern segment is locked from the surface to a depth of 20 km

brittle frictional mountain building 3 low grade metamorphism an active fold and thrust belt in unchanging tectonic and climatic conditions exhibits a dynamic steady state with the flux of rocks accreted at the toe balanced by the flux of rocks eroded off the top rocks entering the toe are buried and heated before they are uplifted and eroded this results in a characteristic map pattern of low grade metamorphism on the surface metamorphic isograds are generally parallel to the regional strike of a fold and thrust belt with the grade increasing progressively from unmetamorphosed and zeolite facies near the deformation front up to greenschist facies in the highest mountains such a pattern is observed in the active fold and thrust belt of taiwan this paper examines the origin of this low grade syntectonic metamorphism using a previously developed mechanical and thermal model of a steady state fold and thrust belt as a basis to model the metamorphism we develop a petrogenetic grid for rocks of basaltic composition considering phases within the chemical system cao mgo al2o3 sio2 h2o equilibria diagnostic of stable low grade mineral assemblages are mapped onto a cross section of the taiwan fold and thrust belt using the calculated fluid pressure and temperature distributions within the wedge the variation of metamorphic grade along the surface is predicted by assuming that there is no retrograde metamorphism the most important mechanical factor controlling the degree of metamorphism in an active fold and thrust belt is the amount of frictional heating both on the basal decollement fault and within the deforming brittle wedge frictional heating raises temperatures in the deepest portions of the taiwan fold and thrust belt by 200 c to 250 c the resulting high temperatures in excess of 400 c are responsible for the extensive greenschist facies metamorphism the amount of heating is well constrained by the observed heat flow anomaly geochronological data and the critical wedge taper the best fitting coefficient of basal friction in taiwan is b 0 5 0 2 underplating of footwall rocks by the assimilation of duplexes along the decollement fault is not considered to be a significant process in taiwan because of the observed balance between the accretionary influx at the toe and the erosive efflux off the top in other fold and thrust belts however underplating can significantly increase the outcrop width of higher metamorphic grades on the surface by increasing the flux of rocks through the higher fluid pressure and temperature regions within the wedge

three dimensional numerical models with varied material properties and erosion rates implications for the mechanics and kinematics of compressive wedges received 19 march 2008 revised 29 january 2009 accepted 13 february 2009 published 21 april 2009 1 we develop three dimensional mechanical models of a compressive wedge and investigate how the form and kinematics of the outboard wedge are affected by variation in initial topography material properties and erosion rate inclusion into the wedge of weaker less dense material affects the form of the wedge producing a region of steeper average slopes and higher topography while depriving the region further inboard of material enhanced erosion has a similar effect the wedge attempts to replace the eroded material by focusing deformation the result is a stepped region of lower topography within the outboard of the orogen we observe that uplift velocities at three points in the orogen vary cyclically from near zero to 3 times the average uplift rate over cycles lasting on the order of 15 200 000 model years our models along with analog models and some well dated examples from active orogens suggest that transient accommodation of strain may be common the cycles observed responded rapidly to changes in the amount of erosion imposed our models suggest that orogens may be driven by strong coupling between erosion and strain on temporal scales of 10 4 10 5 years and spatial scales comparable to the scale of the erosional perturbation we compare our models to the puli embayment of west central taiwan a region of anomalous low topography and suggest that it presence may reflect the presence of weaker and more erodible sediments than those present along strike in the orogen

a midcrustal ramp fault structure beneath the taiwan tectonic wedge illuminated by the 2013 nantou earthquake series 1 taiwan s active mountain belt is a spotlight for orogenic studies and was first used to test the critical taper wedge mechanics the concept of an orogenic wedge above a shallow detachment surface has been highly influential on current understanding of orogenic processes in taiwan however the recent ml 6 2 and ml 6 5 2013 nantou reverse faulting earthquakes in central taiwan have nucleated below the proposed detachment indicating that active mountain building is occurring below the orogenic wedge we estimate the coseismic slip distributions and fault geometry using the uniform stress drop slip inversions the earthquakes occur on essentially the same 30 dipping fault plane ramping up from 20km depth near a cluster of 1999 chi chi earthquake aftershocks to the shallow detachment and the chi chi fault plane the fault could be a deep extension of a mature shallow fault or a newly developed deep ramp fault that is not reflected in the surface geology

a comprehensive relocation of earthquakes in taiwan from 1991 to 2005 we have carried out a comprehensive relocation of a total of 267 210 earthquakes in taiwan that occurred during the past 15 yr we based our relocation process on the earthquake catalog of the taiwan central weather bureau seismic network cwbsn and made improvements in three aspects first we incorporated a large dataset of the s p times from 680 taiwan strong motion instrumentation pro gram tsmip stations distributed throughout the island of taiwan to improve the coverage of earthquakes on the island secondly we added 18 japan meteorological agency jma stations in the southern ryukyu island chain to enhance the station coverage for eastern offshore events especially around the subduction zone northeast of taiwan thirdly we adopted 3d vp and vp vs models in predicting the travel times of p and s waves the effectiveness of these improvements in earthquake re location can be seen in three aspects 1 the reduction in the residuals of p wave arrival times and s p times 2 a better understanding of the attenuation relationship between the peak ground acceleration and epicentral distance and 3 the geologically meaningful patterns of station corrections to p wave arrival times and s p times online material catalog of relocated earthquakes in taiwan from january 1991 to

a thermomechanical wedge model of taiwan constrained by fission track thermochronometry abstract we present 31 new apatite fission track aft ages for the island of taiwan that when combined with existing aft and zircon fission track zft data provide regional spatial coverage of the island with respect to low temperature thermochronometry the overall pattern of zft and aft ages in taiwan exhibits unreset ages in the southern and western portions of the island and reset ages predominantly in the central range and eastern taiwan this pattern supports interpretations of the orogen kinematics as reflecting a crustal scale wedge with a southward propagating collision zone in this model new material is accreted to the wedge from the west and is transferred to the east with the greatest exhumation occurring along the eastern margin as recorded in the reset ages in the east and unreset ages in the west the southward propagating collision is consistent with reset ages in the north where erosional exhumation has been ongoing for longer and unreset ages in the south where the younger collision implies less time for erosional exhumation despite the variation in the age of the collision along the strike of the island the widths of the aft and zft reset age zones remain nearly constant between 23 00 n to 24 00 n and 23 20 n to 24 00 n respectively suggesting that the orogen is in an exhumational steady state over these regions with respect to the aft and zft thermochronometers we use the fission track data in conjunction with observations of crustal structure crustal fabric and heat flow measurements to constrain a time dependent two dimensional thermomechanical model of orogen evolution by accounting for the heat transfer tectonic and erosion processes needed to predict aft and zft ages we are able to investigate the relationship between the measured ages and the tectonic characteristics of the orogen with our model we conclude that 1 roughly half of the material accretion in taiwan occurs through underplating over an approximately 40 km wide region 2 current average erosion rates are 3 3 mm yr in the eastern central range and 2 3 mm yr over the whole island 3 the collision has been propagating southward at a rate between 20 and 51 km ma over the past 2 3 ma and 4 central taiwan is in a topographic thermal and exhumational steady state

dynamic rupture activation of backthrust fault branching we perform dynamic rupture simulations to investigate the possible reactivation of backthrust branches triggered by ruptures along a main thrust fault simulations with slip weakening fault friction and uniform initial stress show that fast propagation speed or long propagation distance of the main rupture promotes reactivation of backthrust over a range of branch angles the latter condition may occur separately from the former if rupture speed is limited by an increasing slip weakening distance towards the junction direction the results suggest a trade off between the amplitude and duration of the dynamic stress near the main rupture front for backthrust reactivation termination of the main rupture by a barrier can provide enhanced loading amplitude and duration along a backthrust rooted near the barrier facilitating its reactivation especially with a high frictional resistance the free surface and depth dependent initial stress can have several additional effects the sign of the triggered motion along the backthrust can be reversed from thrust to normal if a deeply nucleated main rupture breaks the free surface while it is preserved as thrust if the main rupture is terminated by a barrier at depth the numerical results are discussed in relation to several recent megathrust earthquakes in sumatra chile and japan and related topics such as branch feedbacks to the main fault the dynamic view on backthrust fault branching provided by the study fills a gap not covered by quasi static models or observations a specific examined case of antithetic fault branching may be useful for indicating a barrier like behavior along the main fault

kinematics of the tectonic blocks and active faults at the post orogenic stage northern taiwan abstract northern taiwan offers a rare opportunity to quantify the rate and style of deformation in the transition zone from the collision in the southeast subduction rifting to orogenic collapsing in the northwest the updated gps velocity field collected from 2002 to 2014 and leveling vertical velocity field from 2004 to 2010 were inverted by a 3 d block model in this study to infer the tectonic block rotations and fault slip rates in reference to the eurasian plate our optimized modeling results show that the clockwise rotation rate of 20 8 2 8 myr in the hualien block is caused by the coupling of the waning arc continent collision between the luzon volcanic arc and the eurasian plate and the northeastward migration of the block toward the ryukyu trench in ilan domain the rapid clockwise motion was derived at a rate of 34 9 65 9 myr in southern part of the plain we proposed that the rapid clockwise rotation results from the asymmetry rifting at the southwestward extension of the okinawa trough an another new key finding is highlighted in the capability of maximum magnitude 6 2 is indicated on the suao and the choshui faults in the ilan plain

remote triggering of non volcanic tremor around taiwan summary we perform a systematic survey of triggered deep non volcanic tremor beneath the central range cr in taiwan for 45 teleseismic earthquakes from 1998 to 2009 with mw 7 5 and epicentral distance 1000 km to the broad band station tpub triggered tremors are visually identified as bursts of high frequency 2 8 hz non impulsive and long duration seismic energy that are coherent among many seismic stations and modulated by the teleseismic surface waves out of the 45 earthquakes we identified nine teleseismic events associated with nine tremor sources in the southern and five in the northern cr most of the tremor sources are located within the depth range of 15 25 km in the lower crust above the moho we find that the amplitudes of the surface waves play an important role in determining the triggering potential and the apparent triggering threshold is 0 1 cm s 1 or 7 8 kpa however such threshold is partially controlled by the background noise level which could prevent weaker tremor triggered by surface waves with smaller amplitudes from being identified the amplitudes of the triggered tremor show a positive correlation with the amplitudes of the triggering surface waves consistent with the predictions by the clock advance model in addition to amplitudes other factors such as frequency contents and incidence angles also affect the triggering potential we find that intermediate period 30 10 s surface waves could trigger modulate tremors suggesting that long period 30 s surface waves are not always required in long range triggering tremors appear to be triggered by both love and rayleigh waves when the incidence angles are parallel to the strike of the cr all six events triggered tremor primarily during the rayleigh waves for strike normal incidence only the 2001 mw7 8 kunlun earthquake showed predominant love wave triggering this observation can be qualitatively explained by a simple coulomb failure for a left lateral shear on the low angle detachment fault beneath the southern cr

modern vertical deformation rates and mountain building in taiwan from precise leveling and continuous gps observations 2000 2008 1 to characterize the present day vertical displacement field in the active taiwan orogenic belt 1843 precise leveling and 199 continuous gps measurements from 2000 to 2008 are collected and analyzed in this study vertical velocities derived from the leveling data are placed in a reference frame of the chinese continental margin using continuous gps observations at nearby sites the leveling and gps vertical velocities generally reveal a dome shaped pattern with uplift of 0 2 18 5 mm yr in the interior of the mountain range and subsidence on the flanks of the mountains and coastal plains modern uplift rates in the active fold and thrust belt are generally consistent with geologic uplift rates however present day uplift rates in the central range are faster than the million years averaged exhumation rates the modern subsidence rates are generally consistent with geologic rates except for the rates in western coastal areas due to groundwater pumping present day subsidence in the southern central range and northern coastal range is however inconsistent with long term uplift which may reflect interseismic elastic strain accumulation across faults present day subsidence in northern taiwan occurs in a region of postcollisional orogenic collapse we model the present day and geologic vertical velocities and published gps horizontal velocity data across southern taiwan using a 2 d lithospheric model the model suggests a combined slip rate of 40 mm yr on the frontal thrusts and 45 mm yr on the longitudinal valley fault the model requires an additional source of crustal thickening under the central range to match the observed present day uplift rates

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

geomorphic analysis of the central range fault the second major active structure of the longitudinal valley suture eastern taiwan numerous landforms along the longitudinal valley suture of eastern taiwan indicate that two opposing reverse faults currently dominate the suturing process between the luzon volcanic arc and the central range of taiwan the east dipping longitudinal valley fault on the eastern flank of the valley is well known the west dipping central range reverse fault on the western flank of the valley is more obscure nonetheless it has produced many uplifted lateritic fluvial terraces along the eastern flank of the central range in the central reach of the valley from just north of the wuhe tableland to near chihshang the fault appears to be active but blind south of chihshang and inactive along the northern part of the longitudinal valley the late quaternary slip rate of the fault is less than 12 8 mm yr this constraint means that the fault is absorbing far less than half of the horizontal shortening across the longitudinal valley suture however the late quaternary slip rate along the fault may be comparable to the uplift and exhumation rate of the central range this suggests that localized brittle slip along the central range fault is an important component of crustal thickening and uplift of the range even though additional shortening and crustal thickening may be occurring because of pervasive deformation beneath the range

correction to magnetic phases in hemo ilmenite insight from low velocity and high field mossbauer spectroscopy 1 in the paper magnetic phases in hemo ilmenite insight from low velocity and high field mossbauer spectroscopy by c frandsen et al geophysical research letters 34 l07306 doi 10 1029 2006gl029063 2007 the subscripts in the second composition of the following sentence appeared incorrectly the last sentence of paragraph six should read as follows 2 we find the approximate respective compositions of the host and the lamellae are fe0 84mg 2 0 14ti 4 0 98fe 3

a two dimensional dislocation model for interseismic deformation of the taiwan mountain belt we use a global positioning system gps derived surface velocity field of taiwan for the time period between 1993 and 1999 to infer interseismic slip rates on subsurface faults we adopt a composite elastic half space dislocation model constrained by the observed horizontal velocities projected into the direction of plate motion 306 the gps data are divided into northern and southern regions and the velocities in each region are projected into single profiles the model fault geometry includes a shallowly dipping decollement based on the balanced geological cross sections in the coastal plain and western foothills and a two segment fault representing the longitudinal valley fault lvf in eastern taiwan the decollement is composed of two fault segments one extending west under the central range cr and one extending east of the lvf with estimated slip rates of about 35 and 80 mm yr respectively the optimal geometry of decollement is subhorizontal 2 v11 at a depth of 8v9 km the inferred surface location of the western end point of dislocation in the northern profile is located 15 km east of the chelungpu fault while in the southern section it is located beneath the chukou fault the elastic dislocation model successfully matches the horizontal velocity data and predicts elastic strain accumulation in the western foothills that will presumably be released in future earthquakes however considered over multiple earthquake cycles our model cannot explain the topography of the cr and thus fails to predict the active mountain building process in taiwan this failure indicates that both horizontal and vertical velocity fields require a more complex rheological model that incorporates inelastic behavior

mean meridional winds in the mid and high latitude summer mesosphere we examine the mean meridional flow in the summer upper mesosphere using data from the poker flat mst radar and all other available stations the observed mean flow is equatorward at about 10 15 ms 1 near 60 n about an order of magnitude larger than expected from numerical model results the rayleigh friction coefficient required to balance the coriolis torque associated with this flow is about 4 days 1 about ten times larger than values currently used in numerical models

the 2016 mw 6 5 nura earthquake in the trans alai range northern pamir possible rupture on a back thrust fault constrained by sentinel 1a radar interferometry abstract on june 26 2016 an mw 6 5 earthquake occurred in the trans alai range about 37 km west of a 2008 mw 6 7 earthquake both near the town of nura china we use interferometric synthetic aperture radar insar images from sentinel 1a to image coseismic surface displacements from the most recent nura earthquake we invert the interferograms for both fault geometry and slip distribution we find that the insar data can be described by either a steeply north dipping fault or shallowly south dipping fault although only when slip on the south dipping fault is constrained to be blind the insar misfit for the north dipping fault plane is slightly lower and the north dipping fault better describes the gradient of deformation in the near field we prefer the north dipping fault geometry 67 dip and 266 strike as the source for the 2016 nura earthquake not only due to the better fit to the insar but the resulting coseismic slip model is more consistent with the focal mechanism solutions and regional seismogenic thickness however it is important to reiterate that the insar data cannot uniquely determine which model is the definitive representation of the nura earthquake the maximum inferred coseismic slip is about 0 75 m at 14 km depth beneath the trans alai range with the largest slip in the depth range of about 8 20 km slip does not reach the surface and is dominated by thrust displacement with a right lateral strike slip component the geodetic moment of our preferred slip model is 6 61 1018 n m equivalent to mw 6 5 the surface projection of our preferred fault is to the south of the pamir frontal thrust pft and we argue it is a back thrust of the listric pft suggesting that the trans alai range is a pop up structure

two dimensional crustal structures of taiwan from gravity data to delineate the tectonic character of taiwan an island wide gravity survey of taiwan was conducted between 1980 and 1987 the bouguer anomaly map shows that in general isogals trend nne in consonance with the overall structural trend of the island with seismic and other geophysical data as constraints the subsurface density structures were modeled along three profiles across the major structural trends the gravity data are consistent with average continental moho depths of 26 km in the coastal plain and the western foothills 28 km underneath the coastal range in eastern taiwan and 33 km under the central range a lack of significant correlation of the bouguer anomalies with the topography implies dynamic rather than isostatic support of topography in the taiwan region

mechanical constraints on inversion of coseismic geodetic data for fault slip and geometry example from insar observation of the 6 october 2008 mw 6 3 dangxiong yangyi tibet earthquake 1 modern geodetic techniques such as the global positioning system gps and interferometric synthetic aperture radar insar provide high precision deformation measurements of earthquakes through elastic models and mathematical optimization methods the observations can be related to a slip distribution model the classic linear kinematic and static slip inversion problem requires specification of a smoothing norm of slip parameters and a residual norm of the data and a choice about the relative weight between the two norms inversions for unknown fault geometry are nonlinear and therefore the fault geometry is often assumed to be known for the slip inversion problem we present a new method to invert simultaneously for fault slip and fault geometry assuming a uniform stress drop over the slipping area of the fault the method uses a full bayesian inference method as an engine to estimate the posterior probability distribution of stress drop fault geometry parameters and fault slip we validate the method with a synthetic data set and apply the method to insar observations of a moderate sized normal faulting event the 6 october 2008 mw 6 3 dangxiong yangyi tibet earthquake the results show a 45 0 0 2 west dipping fault with a maximum net slip of 1 13 m and the static stress drop and rake angle are estimated as 5 43 mpa and 92 5 respectively the stress drop estimate falls within the typical range of earthquake stress drops known from previous studies

neotectonic characteristics along the eastern flank of the central range in the active taiwan orogen inferred from fluvial channel morphology the tectonic activity of the taiwan orogenic belt has been studied by different methods over various timescales constraints on tectonic characteristics over timescales of 103 105 years however are rather limited to further understand the tectonic forcing over this timescale we analyzed 20 major river basins along the eastern flank of the central range the mountainous core of the orogen the results of river steepness index ksn values and the distribution patterns of knickpoints are consistent with the patterns of tectonic evolution in taiwan on both ends of the orogen the rivers have low ksn values that indicate low uplift rates throughout the area in the central part of the island where the orogen is under rapid collisional processes the rivers are steeper and are characterized by many tectonic knickpoints our results show different patterns from both short term and long term uplift and denudation rate patterns this suggests that these rates change temporally and spatially moreover the steepness indexes of rivers in the taiwan orogen show the same increasing trend with erosion rates as those in other active orogens but correspond to significantly higher erosion rates this is likely produced by the frequent debris flow and landslide events in taiwan a character that makes the island distinctive from other active orogens in the world

active fault creep variations at chihshang taiwan revealed by creep meter monitoring 1998 2001 1 the daily creep meter data recorded at chihshang in 1998 2001 are presented the chihshang creep meter experiment was set up across the chihshang thrust fault the most active segment of the longitudinal valley fault which is the present day plate suture between the eurasian and the philippine sea plates in eastern taiwan near continuous data recording at two sites revealed different surface fault motions yet similar annual shortening rates 16 2 mm at the tapo site comprising two connected creep meters and 15 0 mm at the chinyuan site three creep meters straddling parallel fault branches four of the five creep meters showed a seasonal variation with the fault moving steadily during the rainy season from april to october and remaining quiescent during the rest of the year the only exception was recorded by the creep meter located on a melange composed hillslope where local gravitational landsliding played an additional role other than tectonic faulting through comparison with daily precipitation data we inferred that moderate rainfall suffices to trigger or facilitate slippage on the surface fault during the transition period of the dry wet season during the observation period from 1998 to 2001 the subsurface seismicity exhibited clusters of microearthquakes on the chihshang fault at depths of 10 25 km recurrent earthquakes occurred regardless of whether the season was wet or dry indicating that the stress relaxation associated with seismicity in the seismogenic zone did not transfer immediately up to the surface the accumulated strain on the chihshang fault at shallow surface levels was released through creep during the wet season in addition to these short term seasonal variations an apparent decrease in the annual slipping rate on the chihshang fault during the last few years deserves further investigation in order to mitigate against seismic hazard

incision and channel morphology across active structures along the peikang river central taiwan implications for the importance of channel width river morphology and dynamics are strongly influenced by active tectonics we report channel dynamics for the peikang river which flows through the hsuehshan range in central taiwan using a digital elevation model and field surveys we constrain channel morphology for an 90 km stretch of river to calculate unit stream power and boundary shear stress along the river path incision rates are estimated with optically stimulated luminescence dating of sand deposited on strath terraces we find a strong correlation between unit stream power shear stress and incision rate but only if variation in channel width is considered a calibrated river incision rule implies river incision rates of 9 13 5 mm yr upstream of the meiyuan and tili faults and suggests that one or both of these structures are presently active our results indicate that the shuilikeng fault is also actively deforming as incision rates increase to 6 10 mm yr across it compared to 1 4 mm yr in adjacent reaches prominent narrowing across the shuilikeng fault and the absence of significant gradient variation indicate that channel width is a first order morphological adjustment to differential incision only when the channel width to depth ratio reaches a minimum does the channel slope significantly adjust to local changes in base level as is the case upstream of the meiyuan and tili faults

the influence of backstop dip and convergence velocity in the growth of viscous doubly vergent orogenic wedges insights from thermomechanical laboratory experiments abstract we investigate the effects of convergence velocity and backstop geometry on the evolution of experimental viscous doubly vergent orogenic wedges experiments are performed in a thermomechanical shortening box using the newtonian properties of 52 54 en type paraffin as analogue to the natural strength distribution in the continental crust a velocity discontinuity is imposed at the subduction slot where the base plate is pulled below a rigid backstop the mobile base plate dips 5 two different backstop dips 60 and 30 are used in combination with constant convergence velocities of 2 and 10 cm h 1 results point out that backstop geometry exerts a first order control on the evolution of doubly vergent experimental wedges steeply dipping backstops cause the stationary uplift of the wedge axial region above the subduction slot whereas shallow dipping backstops allow its retroward translation in concomitance with a more pronounced deformation in the retrowedge convergence velocity controls the uplift in the wedge axial region and the amount of the outward migration of the deformation front in both the prowedge and retrowedge regions for fast convergence velocity and shallow dipping backstop deformation in the retrowedge is favoured an increase in the backstop dip and or a decrease in the convergence velocity favour deformation in the prowedge both prowedge and retrowedge tend to grow in self similar conditions with an aspect ratio depending upon the backstop dip and convergence velocity these results may have important consequences for the evolution and deformation style of ductile dominated orogenic wedges suggesting that location of the axial region and deformation progression in natural orogens can be strongly controlled by boundary and kinematic conditions

vhf radar observations of the dynamics of the summer polar mesopause region observations of the dynamics of the mesopause region over the norwegian island of andoya 69 n 16 e made during 2 days in summer 1987 using the mobile sousy vhf 53 5 mhz doppler radar are reported measurements of the three mean wind components and the 12 and 24 hour horizontal tidal components as well as preliminary estimates of the component of the total upward flux of density normalized horizontal momentum in the sw ne plane and the anisotropy of the horizontal wind are presented for the 83 to 90 km height interval values of the component of the density normalized flux in the sw ne plane for the 10 min to 48 hour period range after the 12 and 24 hour tidal components have been removed vary from 1 m2 s 2 near 84 km to 16 m2 s 2 near 89 km the major contribution is from the 10 min to 12 hour period range the corresponding mean flow acceleration takes a mean value of 136 m s 1 d 1over the 83 90 km height interval the anisotropy is considerable and may suggest the presence of standing waves

brittle frictional mountain building 1 deformation and mechanical energy budget an active fold and thrust belt is analogous to a wedge of soil or snow that forms in front of a moving bulldozer such wedges exhibit a critical taper and a regional state of stress that is everywhere on the verge of coulomb failure the width of such a critically tapered fold and thrust belt does not depend on its brittle strength or frictional properties but rather on the accretionary influx rate of fresh material at its toe and on the rate of erosion a steady state fold and thrust belt is one in which the accretionary influx is balanced by the erosive efflux rocks are accreted at the toe and then horizontally shortened as they are transported toward the rear those that enter lower in the accreted section are more deeply buried before being uplifted by erosion mass balance and isotropy constrain the kinematics of this large scale deformation enabling us to infer the trajectories residence times and stress strain histories of rocks incorporated into eroding fold and thrust belts a typical rock resides in the steady state taiwan wedge for 2 3 m y before it is uplifted and eroded during its motion through the wedge it experiences strain rates in the range 10 13 to 10 14 s 1 the mechanical energy budget of brittle frictional mountain building is described by the equation where is the rate of work performed on the base and front of the fold and thrust belt by the subducting plate is the rate at which energy is dissipated against friction on the decollement fault is the rate at which energy is dissipated by internal frictional processes within the deforming brittle wedge and is the rate of work performed against gravitational body forces in a reference frame attached to the overriding plate the total mechanical power being supplied by the subducting eurasian plate to the active fold and thrust belt in taiwan is slightly over 3 gw approximately 60 of this work of steady state mountain building is being dissipated against friction on the decollement fault and about another 25 is being dissipated against internal friction this leaves only 15 or roughly half a gigawatt available to do useful work against gravity in general fold and thrust belts with moderate pore fluid pressures are dominated by work done against friction on the decollement fault however those with nearly lithostatic pore fluid pressures may be dominated by work done against gravity internal frictional dissipation is always less than basal frictional dissipation as it is in taiwan an alternative and equivalent description of the mechanical energy balance of a steady state fold and thrust belt is provided by the equation in this version the quantity on the left is the rate at which work is performed on the back of the wedge by the overriding plate and is the rate of work performed against gravity in a reference frame attached to the subducting plate the latter quantity is always positive for any critically tapered fold and thrust belt whose decollement fault dips toward its rear in contradiction to the central premise of the gravity spreading theory of fold and thrust tectonics

gravity wave radiation and mean responses to local body forces in the atmosphere the authors determine the spectral linear solutions that arise in response to local 3d body forces and heatings in an idealized environment that turn on and off smoothly but not necessarily slowly over a finite interval in time the solutions include impulsive through slowly varying body forcings the forcings result in both a mean response which is typically significantly broadened spatially in one direction and a gravity wave response which allows the fluid to reach this state the gravity wave field depends on both the spatial attributes of the source and the forcing duration the frequency of the wave response is the characteristic source frequency formed from the source dimensions if the forcing frequency is greater than the characteristic frequency and is the forcing frequency otherwise the radiated gravity waves from zonal forcings have vertical wavelengths which are approximately twice the vertical extent of the forcing and horizontal wavelengths which are at least twice the horizontal extent of the forcing wave excitation is increasingly inefficient when the forcing frequency is smaller than the characteristic source frequency in addition the mean responses are not confined to the source region in general significant spatial broadening of the mean responses occurs if the source s frequency is high and low the responses are broadened horizontally and vertically respectively with the amount depending on the characteristic scales of the source if the body forcing is in the eastward direction then much or all of the ensuing zonal mean wind is eastward however for many realistic forcing scenarios a large percentage of the ensuing zonal wind flows westward these countersigned jets are displaced meridionally about the source thus spatially confined body forcings create both gravity wave and mean responses if the forcings are fast enough very slowly varying forcings create only mean responses

indentation of the philippine sea plate by the eurasia plate in taiwan details from recent marine seismological experiments abstract we analyze in this study a new set of marine data including 3d local tomography 1992 2008 relocated earthquakes and two recent multichannel seismic lines to characterize the deformation style in the collision area offshore east taiwan we have mapped in detail the mohos of the converging plates as well as the subduction interface with a resolution never reached before we show that the sharp continental subduction of the eurasia plate beneath the middle part of the central range indents the philippine sea plate psp as attested by intra oceanic slicing and incipient subduction of the psp beneath the east coast of taiwan the westernmost part of the psp slab is probably experiencing a beginning of break off as attested by nw trending en echelon shear zones beneath the southern slope of the southern ryukyu arc sra these en echelon shear zones have a sinistral component favored by the collision free subduction of the psp north of 24 30 n the down faulting of the subduction interface forms ramps along which earthquakes clusterize three m7 subduction earthquakes occurred offshore suao city along these ramps with a recurrence interval of about 40 years 1920 m w 7 7 1963 m w 7 2 and 2002 m w 7 1 events the 1966 m w 6 0 7 5 earthquakes sequence likely outlines a wnw ese left lateral intra slab shear zone the sra upper plate accommodates the complex geometry and deformation of the subducting psp through seismic deformation shallow high velocities fringing the luzon volcanic arc lva beneath the longitudinal valley and north of the southernmost ryukyu forearc basins are interpreted as relics of the lva forearc basement squeezed in the collision zone based on the accommodation of a large part of the convergence through shortening within the psp and the subsequent segmentation of the shallow subduction interface we consider that the nucleation of a m w 8 earthquake along the southernmost ryukyu megathrust is unlikely

cleavage fronts and fans as reflections of orogen stress and kinematics in taiwan recent observations of cleavage patterns strain histories and kinematics across the taiwan mountain belt depict systematic orogen scale variations with respect to the synorogenic divide and suggest that the pattern of cleavage development is a predictable consequence of orogen stresses and kinematics in taiwan continental crust within the collision is accreted in the prowedge facing asia but is advected eastward into the east verging retrowedge where the most deeply exhumed rocks are exposed wedge mechanics predict a reversal in the direction of plunge of the principal compressive stress at the topographic divide between the opposing wedges the observation of a single cleavage in western taiwan suggests that the cleavage in the prowedge remains stable with respect to the stress orientation in contrast the existence of a second crenulation cleavage in the retrowedge is evidence for an abrupt change in stress orientation and unstable buckling of preexisting prowedge fabrics advection of a fabric across a topographic divide in a doubly vergent wedge provides an explanation for the occurrence of cleavage fronts and fans in natural systems such as taiwan

three dimensional p velocity structures of the lithosphere beneath taiwan from the analysis of taiger and related seismic data sets 1 the convergence of the philippine sea and the eurasian plates in the taiwan region led to the formation of a young collisional mountain between two subduction zones of nearly orthogonal polarities the geological processes underlying the collision and its relation to subduction are the primary targets of taiger taiwan integrated geodynamic research project newly acquired passive and active sources data on land supplemented by ocean bottom as well as permanent seismic network data are used to derive a new 3 d tomographic velocity model using the 7 5 km sec contour as a marker for crustal deformation two trend parallel roots one under the central range with a maximum depth of 55 km and the other under the coastal range at 40 km are found to extend from southern to central 24 n taiwan between the two roots and lying approximately beneath the longitudinal valley the 7 5 km sec contour rises to 25 km depth in the upper mantle a high velocity zone east dipping and its upper surface coinciding with a wadati benioff zone in the south 22 8 n becomes near vertical and ill defined in the north 23 9 n the crustal deformation as defined by the roots and the along trend variations of the upper mantle structures under taiwan provide key information for the orogeny with the thickening of crust to 55 km or more processes such as eclogitization and delamination may come into play

magnetization of exsolution intergrowths of hematite and ilmenite mineral chemistry phase relations and magnetic properties of hemo ilmenite ores with micron to nanometer scale lamellae from allard lake quebec hemo ilmenite ores from allard lake quebec were first studied over 50 years ago interest was renewed in these coarsely exsolved oxides based on the theory of lamellar magnetism as an explanation for the high and stable natural remanent magnetizations nrms 32 to 120 a m reported here to understand the magnetism and evolution of the exsolution lamellae the microstructures and nanostructures were studied using scanning electron microscopy and transmission electron microscopy tem phase chemistry and relations between mineral chemistry and the hematite ilmenite phase diagram cycles of exsolution during slow cooling resulted in lamellae down to 1 2 nm thick combined electron microprobe tem and x ray diffraction xrd results indicate that hematite hosts reached a composition approximately ilmenite ilm 14 4 and ilmenite hosts ilm 98 the bulk of the very stable nrm which shows thermal unblocking 595 620 c was acquired during final exsolution in the two phase region canted antiferromagnetic r3c hematite r3 ilmenite hysteresis measurements show a very strong anisotropy with a stronger coercivity normal to than parallel to the basal plane orientation of the lamellae magnetic saturation m s values are up to 914 a m compared to 564 a m predicted for a modally equivalent spin canted hematite corrected for 15 r 2 tio 3 substitution low temperature hysteresis ac susceptibility measurements and mossbauer results indicate a neel temperature t n of the geikielite substituted ilmenite at 43 k the low temperature hysteresis and ac susceptibility measurements also show a cluster spin glass like transition near 20 k below t n of ilmenite an exchange bias occurs with a 40 mt shift at 10 k

improved seismic tomography offshore northeastern taiwan implications for subduction and collision processes between taiwan and the southernmost ryukyu summary to improve the resolution in imaging the 3 d vp and vp vs structures in the taiwan region especially offshore eastern taiwan we combine the arrival times from eleven ocean bottom seismometers obss and from seismic stations of the japan meteorological agency jma with those from the permanent stations of taiwan central weather bureau seismic network cwbsn and taiwan strong motion instrumentation program tsmip by doing so we have obtained a new 3 d tomographic model for taiwan and its surrounding regions with a better resolution especially in the area offshore northeastern taiwan we also used this new tomography model to relocate the hypocentres of the earthquakes in northeastern taiwan and determined the focal mechanisms of relatively large events our tomography results indicate that in the region northeast of taiwan the subducting oceanic philippine sea plate beneath the eurasian continental lithosphere is characterized by a high vp layer surrounded by lower vp areas this p wave velocity characteristics of the subducting slab provides a better constraint on the geometry of the subduction interface especially in its shallower portion in the hangingwall eurasian block above the subduction interface a vertically elongated high vp vs body appears to originate from the interface at depths between 100 and 140 km we suggest that it represents the partially melted materials that are related to the magmatic activity of the ryukyu volcanic arc

erosion rates and orogenic wedge kinematics in taiwan inferred from fission track thermochronometry new apatite and zircon fission track ages and previously published thermochronometric data are used to evaluate erosion rates and particle paths within the active taiwan arc continent collision we present 20 new apatite fission track ages and 6 new zircon fission track ages apatite and zircon ages are all reset in the northern and eastern parts of taiwan although the region of reset apatite ages is larger we interpret this pattern as resulting from crustal accretion at the western margin of the orogenic wedge combined with southward propagation of the collision zone a onedimensional thermal model including erosion provides prediction of the fission track ages the distribution of reset ages is best explained with an erosion rate of 4 6 mm yr given a propagation velocity of 60 mm yr this erosion rate implies that nearly 25 km of material has been eroded from northern taiwan the lack of reset 40 ar 39 ar ages from muscovite and biotite suggests that rockparticle paths have a large horizontal component a result consistent with an eroding orogenic wedge model

neotectonic architecture of taiwan and its implications for future large earthquakes the disastrous effects of the 1999 chi chi earthquake in taiwan demonstrated an urgent need for better knowledge of the island s potential earthquake sources toward this end we have prepared a neotectonic map of taiwan the map and related cross sections are based upon structural and geomorphic expression of active faults and folds both in the field and on shaded relief maps prepared from a 40 m resolution digital elevation model augmented by geodetic and seismologic data the active tandem suturing and tandem disengagement of a volcanic arc and a continental sliver to and from the eurasian continental margin have created two neotectonic belts in taiwan in the southern part of the orogen both belts are in the final stage of consuming oceanic crust collision and suturing occur in the middle part of both belts and postcollisional collapse and extension dominate the island s northern and northeastern flanks both belts consist of several distinct neotectonic domains seven domains kaoping chiayi taichung miaoli hsinchu ilan and taipei constitute the western belt and four domains lutao lanyu taitung hualien and ryukyu make up the eastern belt each domain is defined by a distinct suite of active structures for example the chelungpu fault source of the 1999 earthquake and its western neighbor the changhua fault are the principal components of the taichung domain whereas both its neighboring domains the chiayi and miaoli domains are dominated by major blind faults in most of the domains the size of the principal active fault is large enough to produce future earthquakes with magnitudes in the mid 7 values