vp vs distribution in the northern taiwan area implications for the tectonic structures and rock property variations abstract the ratio of compressional wave velocity vp to shear wave velocity vs is often indicative of lithology and rock conditions in this study we determine the vp vs characteristics for the different tectonic features in northern taiwan and quantify the crack density and saturation by using the ob74 method the result shows that the highest vp vs values are located beneath the western foothills at depths between 4 and 21 km these vp vs highs are simultaneously characterized by low vp and low vs suggesting the presence of fluids the average crack density and saturation for each depth can reach as high as 25 and 91 at depths of 7 and 11 km respectively this distribution could be caused by the presence of less compacted sediments within the foreland basin and the progressive convergence of crustal faults with depth as the shallow portion of the foreland basin is highly fractured fluids could infiltrate into deeper parts to create a highly saturated environment however when the depth is deeper than 11 km the high crack density and saturation area migrate from the west to the east of the shanchiao fault this observation may indicate the presence of a preexisting syn rift basin buried at depth during the collision in the ilan plain the highest crack density and saturation are observed in its northeastern portion at a depth of 4 km with values of 17 1 and 60 3 respectively however slightly high saturation values spread throughout the plain at a depth of 15 km which may indicate a possible fluid conduit from a deep hot rock body to the surface and could be applied toward studies on geothermal fluid activity

three dimensional p and s velocity structure in the coalinga region california the coalinga earthquake sequence of 1983 provided a unique opportunity to perform a three dimensional velocity and hypocenter inversion in an area of complex three dimensional structure dominated by folding and blind thrusts additionally since other varied geological and geophysical studies have been completed in this area the three dimensional inversion solution could be compared to other interpretations inversion of 7696 p and 1511 s first arrivals from earthquakes and 696 p first arrivals from refraction shots produced a three dimensional velocity model with grid spacing of 1 2 km in the hypocentral area the overall shape and location of velocity features correspond well to the mapped surface geology the three dimensional inversion yields details of folds where the resolution is good and the general shape where resolution is lower the amounts of structural relief inferred for the local folds are similar to values inferred from geologic data the three dimensional velocity solution has several distinctive features there is a linear high velocity body 6 1 6 5 km s about 25 km long from 6 to 8 km depth that may represent a fragment of coast range ophiolite a shallow low velocity zone lvz which extends for 20 km along the fold axis at about 6 km depth and correlates with lvzs observed in both the refraction and the reflection data may indicate high pore pressure caused by lateral compressive strain deeper lvzs occur within inferred franciscan material and are characterized by horizontal or southwest dipping zones of varied thickness 4 8 km wide and 5 10 km long these lvzs may represent multiple thrust faults their locations and geometry are consistent with thrust faults inferred with seismic reflection data the three dimensional velocity solution compares well to prior two dimensional seismic reflection and refraction models and observed gravity the shape of the inferred sedimentary section agrees well with the reflections from the cenozoic strata compared to the refraction model the three dimensional solution has similar velocities and similar locations of velocity features but is more detailed in the hypocentral zone where it uses more data the gravity computed from the three dimensional velocities is similar to the observed gravity in both shape and amplitude both a simple one dimensional initial model and a complex initial model derived from the refraction interpretation were tried a simple starting model gave the best results the s velocity solution has different resolution than the p velocity solution because it uses a different set of stations and it has lower resolution because it uses fewer arrival times while the general patterns of velocity variation are similar for both vp and vs the vs solution tends to have more smearing of velocity features and can have somewhat different locations of velocity features

reynolds stress and sand transport over a foredune reynolds shear stress rs u w and sand transport patterns over a vegetated foredune are explored using three dimensional velocity data from ultrasonic anemometers at 0 2 and 1 2 m and sand transport intensity from laser particle counters at 0 014 m a mid latitude cyclone on 3 4 may 2010 generated storm force winds exceeding 20 m s 1 that shifted from offshore to obliquely alongshore quadrant analysis was used to characterize the spatial variation of rs quadrant components q1 through q4 and their relative contributions were parameterized using the flow exuberance relation exfl q1 q3 q2 q4 the magnitudes of rs and sand transport varied somewhat independently over the dune as controlled by topographic forcing effects on flow dynamics a flow exuberance effect was evident such that q2 ejection like and q4 sweep like quadrants that contribute positively to rs dominated on the beach dune toe and lower stoss whereas q1 and q3 that contribute negatively to rs dominated near the crest this exuberance effect was not expressed however in sand transport patterns instead q1 and q4 with above average streamwise velocity fluctuations u were most frequently associated with sand transport q4 activity corresponded with most sand transport at the beach toe and stoss locations 52 60 100 at the crest 25 to 86 of transport was associated with q1 while q4 corresponded with most of the remaining transport 13 to 59 thus the relationship between sand transport and rs is not as straightforward as in traditional equations that relate flux to stress in increasing fashion generally rs was poorly associated with sand transport partly because q1 and q4 contributions offset each other in rs calculations thus large amounts of transport can occur with small rs turbulent kinetic energy or reynolds normal stresses u 2 w 2 may provide stronger associations with sand transport over dunes although challenges exist on how to normalize and compare these quantities copyright 2013 john wiley sons ltd

source location dependency site response in the taipei basin of taiwan by using hvsr analysis abstract techniques using horizontal to vertical spectral ratios hvsr have frequently been adopted in many studies particularly of seismic site amplification however the influence of source location on the hvsr is not previously well studied this paper studies the variation of the s wave hvsr in the taipei basin based on a variety of hypocenter locations the earthquake records are classified into three categories include depth azimuth and distance we report the s wave hvsr to have strong focal depth dependence for which the hvsr of the deep events are larger than shallow events besides the s wave hvsr in the northern and northeastern parts of the basin also displays source azimuthal dependence however there is no apparent difference in source distance dependence in contrast to the s wave hvsr the coda wave hvsr all showed no apparent difference for various earthquake hypocenter locations the synthetic modeling results support our idea that the cause of the variation of hvsr results for different earthquake focal depths is the different s wave angles of incidence into the basin based on the dependency of the site amplification on the focal depth a focal depth dependent empirical transfer function etf was established the focal depth dependent etf was verified to improve the accuracy of the ground motion prediction by using the stochastic method in this study we demonstrate that the source location effect especially the focal depth can significantly influence the s wave hvsr and therefore needs to be taken into account in site response investigation

crustal deformation at the southernmost part of the ryukyu subduction east taiwan as revealed by new marine seismic experiments abstract the southernmost part of the ryukyu subduction where the philippine sea plate is subducting under the eurasian plate is known to be a very seismically active region of transition from a north dipping subduction along the ryukyu subduction to an se nw collision along the taiwanese orogenic wedge in this paper we will focus on the ryukyu forearc area close to taiwan where the deformation is paroxysmal in order to decipher the nature of the seismic deformation in this region a three month passive experiment combining 22 ocean bottom seismometers and 51 onland stations has been led starting from an a priori heterogeneous model we have obtained 801 well located earthquake hypocenters a precise p wave tomography model and 14 focal mechanisms the seismicity along the ryukyu forearc is mainly located not only in the vicinity of the interplate seismogenic zone isz but also within both the subducting psp and the overriding plate seismicity within the upper plate is essentially localized east of nanao basin where nw se extension occurs and northwest of the hoping basin where strike slip dominates as revealed by both the p wave velocity structure and the newly derived seismicity we argue that a sub vertical step offsetting the subducting psp around 10 km may support the presence of a trench parallel tear the psp also undergoes extension in its upper part that is probably caused by buckling and slab pull the p wave velocity structure reveals three other major features 1 a continuity between the central range and the ryukyu arc with a shallower moho 30 km depth between 122 3 n and 122 5 n along the ryukyu arc 2 high p wave velocities along the eastern side of the central range and 3 two bodies with similar high crustal velocities 6 5 7 0 km s at 12 18 km depths embedded within the ryukyu arc basement just north of hoping basin and north of the nanao basin

a further study of the tropical western hemisphere warm pool abstract variability of the tropical western hemisphere warm pool whwp of water warmer than 28 5 c which extends seasonally over parts of the eastern north pacific the gulf of mexico the caribbean and the western tropical north atlantic tna was previously studied by wang and enfield using the da silva data from 1945 93 using additional datasets of the ncep ncar reanalysis field and the ncep sst from 1950 99 and the levitus climatological subsurface temperature the present paper confirms and extends the previous study of wang and enfield the whwp alternates with northern south america as the seasonal heating source for the walker and hadley circulations in the western hemisphere during the boreal winter a strong hadley cell emanates northward from the amazon heat source with subsidence over the subtropical north atlantic north of 20 n sustaining a strong north atlantic anticyclone and associated northeast ne trade winds over its southern limb in the tna this circulation including the ne

the taiwan ryukyu subduction collision complex folding of a viscoelastic slab and the double seismic zone 1 the termination of the ryukyu trench against eurasia and the oblique subduction of the philippine sea plate create a subduction collision complex offshore taiwan which has not previously been elucidated in detail we combine traveltime data from the seismic networks in taiwan and japan to better illuminate how the subducting ryukyu slab deforms in this subduction collision zone more than 5000 events recorded by both networks were relocated with the double difference method using an optimal regional one dimensional velocity model the offshore seismicity indicates that the double seismic zone with a gap of 15 20 km exists in the subducting slab in the depth range of 40 80 km focal mechanisms suggest that the double seismic zone is caused by east west compression resulting from oblique convergence the improved hypocentral locations for the first time reveal folding of the slab into a horizontal curvature larger in magnitude than and opposite in sign to that of the ryukyu trench in the depth range 50 100 km the anomalous curvature together with the focal mechanisms suggests that the slab folds against the eurasian lithosphere and that this deformation cannot be fully elastic we model this deformation mode as the developing instability of a viscoelastic maxwell layer embedded in a viscous medium the characteristic wavelength of the instability i e 250 km is consistent with folding of a slab whose viscosity is 100 times higher than that of the surrounding mantle for an along strike elastic membrane strain as small as 0 01 or more than 3 orders of magnitude higher if 5 elastic strain is allowed

numerical simulation of saltating particles in atmospheric boundary layer over flat bed and sand ripples 1 in this work we numerically simulated the saltating particles in a turbulent boundary layer over flat bed and sand ripples by using natural sand grains in a wind tunnel we obtained the initial conditions for the simulation and also verified the correctness of the numerical model we carefully analyzed the numerically simulated saltating particle movement over the two sand beds and we found the following 1 the aeolian sand transport is a dynamic equilibrium process on both sand beds and it took longer to reach equilibration on the sand ripples than on the flat bed 2 according to the mass flux profile at the trough of the sand ripples there is a maximum mass flux at about 4 cm height in the leeward section the mass flux increases with height below 4 cm and decreases with height above 4 cm 3 the wind profile near the surface is modified by saltating particles on the two different sand beds and the flow field characteristics of the sand ripples are more complex than that of the flat bed

crustal deformation and block kinematics in transition from collision to subduction global positioning system measurements in northern taiwan 1995 2005 1 we present global positioning system gps measurements for the period 1995 2005 at 125 campaign surveyed sites in northern taiwan based on elastic rotating block modeling analyses derived from the gps data we describe the transitional tectonics from arc continent luzon chinese collision to the converging ryukyu trench subduction and back arc opening along the chinese continental margin station velocities relative to station s01r in the chinese stable continental margin were estimated from coordinate time series of each station by using the weighted least squares technique we found two distinct deformation patterns in two geological areas which are basically separated by the surface projection of the nw trending boundary of the subducting philippine sea plate across northern taiwan 1 a waning collision area to the west and 2 a transition zone to the east in the waning collision area the horizontal velocity field shows vectors of 0 3 7 3 mm yr toward the nw in the foothills and the hsuehshan range of northwestern taiwan the tectonic blocks represent a significant nw se internal contraction along with a small block rotation rate 3 0 myr the transition zone can be further divided into an outer range and inner range with distinguishing rotation rates and deformation behaviors in the outer range of the transition zone velocities of 1 0 7 8 mm yr from south to north rotating from 008 to 143 is found in the northernmost foothills and the hsuehshan range the tectonic blocks within the outer range are characterized by a coherent rotation low internal strain rate of 0 10 strain yr with an angular velocity of 5 1 myr where the euler pole is located near its southeastern boundary in the inner range of transition zone a larger clockwise rotation from west to east with horizontal velocities of 9 3 41 2 mm yr from 053 to 146 are found in the northernmost central range the tectonic blocks of the inner range reveal a remarkable nw se internal extension with an ultrarapid clockwise rotation 47 3 myr where the euler pole is near the southern boundary of the range close to the collision comer with the colliding luzon arc the trench roll back together with back arc opening are interpreted to be substantially superposed on the arc continent collision induced rotation in the transition zone with particular regard to the inner range of the northeast taiwan mountain belt

empirical relationships between aftershock zone dimensions and moment magnitudes for plate boundary earthquakes in taiwan in this study we establish the empirical relationships between the spatial dimensions of the aftershock zones and moment magnitudes m w for the taiwan region the length l and width w of the aftershock zone of an earthquake is measured by the major and minor axes respectively of the ellipse of a two dimensional gaussian distribution of one day aftershocks our data is composed of 649 mainshocks depth 70 km m w 4 0 7 6 between 1990 and 2011 the relationships between aftershock zone dimensions and m w were obtained by least squares method with the corresponding uncertainties estimated by bootstrap our study confirms that aftershock zone dimensions are independent of faulting types and the seismic moment is proportional to l 3 the ratio w l increases slightly with m w and is independent of faulting types together with previous study our results suggest that earthquakes of both small m w 4 0 and large m w 7 0 magnitudes have similar focal zone geometrical parameters by using the m w s relation where the aftershock zone area s is estimated from l and w we also provide an independent examination of the variations in the median stress drop we find that the median stress drops of strike slip earthquakes are higher than those of thrust events moreover the median stress drops are independent of the moment magnitudes for normal and strike slip events but decrease for large thrust events these results are consistent with the latest global observations regardless of faulting types the median stress drop decreases for larger 6 m w 7 6 and relatively deep depth 60 70 km earthquakes online material effect of up dip rupture propagation on estimated maximum slip duration details on teleseismic and geodetic stations and figures of far field displacements temporal change in slip and snapshots of the semblance value distribution

a field study of mean and turbulent flow characteristics upwind over and downwind of barchan dunes field measured patterns of mean velocity and turbulent airflow are reported for isolated barchan dunes turbulence was sampled using a high frequency sonic anemometer deriving near surface reynolds shear and normal stresses measurements upwind of and over a crest brink separated barchan indicated that shear stress was sustained despite a velocity reduction at the dune toe the mapped streamline angles and enhanced turbulent intensities suggest the effects of positive streamline curvature are responsible for this maintenance of shear stress this field evidence supports an existing model for dune morphodynamics based on wind tunnel turbulence measurements downwind the effect of different dune profiles on flow re attachment and recovery was apparent with transverse incident flow a re attachment length between 2 3 and 5 0h h is dune brink height existed for a crest brink separated dune and 6 5 to 8 6h for a crest brink coincident dune the lee side shear layer produced elevated turbulent stresses immediately downwind of both dunes and a decrease in turbulence with distance characterized flow recovery recovery of mean velocity for the crest brink separated dune occurred over a distance 6 5h shorter than the crest brink coincident form as the application of sonic anemometers in aeolian geomorphology is relatively new there is debate concerning the suitability of processing their data in relation to dune surface and streamline angle this paper demonstrates the effect on reynolds stresses of mathematically correcting data to the local streamline over varying dune slope where the streamline angle was closely related to the surface windward slope time averaged shear stress agreed best with previous wind tunnel findings when data were rotated along streamlines in the close lee however the angle of downwardly projected separated flow was not aligned with the flat ground surface here shear stress appeared to be underestimated by streamline correction and corrected shear stress values were less than half of those uncorrected

aeolian sediment transport on a beach thresholds intermittency and high frequency variability abstract during a field experiment designed to measure wind flow and sediment transport over the beach and foredune at greenwich dunes prince edward island national park measurements were made on october 11 2004 during a storm with wind speeds ranging from 4 ms 1 to over 20 ms 1 this paper examines thresholds of sand movement intermittency and the relationship between fluctuating winds and transport intensity based on high frequency measurements of wind speed and saltation wind speed was measured at four points across the beach profile using cup anemometers set at a height of 0 6 m the anemometers were co located with saltation probes set at a height of 0 02 m that measure the impacts of saltating sand grains the instruments were sampled at 1 hz during runs of 1 or 2 h over an eight hour period sand transport was highly intermittent early in the day when winds were only slightly above threshold and became more continuous towards the upper beach because of increasing fetch distance and decreasing surface moisture despite more continuous transport and increasing wind speed through the day a zone of highly intermittent transport migrated landward across the beach as water levels rose due to storm surge examination of three thresholds related to measures of wind speed the maximum wind speed without transport u tmax minimum wind speed with transport u tmin and the intermittency threshold u t showed that none provide a consistently robust measure of the threshold although they all provide some insight into conditions controlling entrainment while graphs of instantaneous saltation intensity crudely mimic the pattern of fluctuating wind speed statistical correlations over periods of 2 10 min were generally poor r 2

turbulence burst over four micro topographies in the wind tunnel abstract investigating the mechanisms of wind sand movement is essential to deeply understand sand storm and desertification process and further to control the wind sand hazards it is considered that turbulence burst may have great influence on the motion of sand particles but current theoretical and experimental studies mostly concentrate on the turbulence burst over flat surface or in the water sand dunes and sand ripples are basic forms of desert landscape and the slope gradient will inevitably affect the frequency of turbulence burst in this paper instantaneous velocities over 4 different sand surfaces including smooth flat bed flat sand bed sand bed with ripples and slope bed are measured using one dimensional hot wire detector the results show that for the slope sand surface wind field in leeward slope region is a buffering zone which doesn t obey logarithmic function on the flat sand surface the distribution of turbulence intensity and reynolds normal stress are gradually decreasing along the height of boundary layer on the slope sand surface the stronger turbulence behavior happens near the top of slope based on mu level method analysis it is found that the turbulence bursts on both flat sand surface and sand ripples mainly occur near the surface and gradually decrease with the height compared to flat sand surface the burst frequency is 30 50 higher on the sand ripples surface which indicates that micro topography have an influence on the turbulence burst this work may be helpful in understanding the initiation and transportation of sand storm as well as the desertification process

three dimensional vp and vs structural models associated with the active subduction and collision tectonics in the taiwan region summary 3 d vp and vs models for the crust and upper mantle beneath the taiwan area have been determined using selected high resolution earthquake data from an island wide seismic network and two local seismic arrays lateral structural variations in the upper crust as also evident from surface geology are responsible for the observed large traveltime residuals or station corrections prior shallow velocity information inferred from traveltime residuals and joint hypocentral determination jhd station corrections for the uppermost crust is essential to facilitate a reliable tomographic inversion a finite difference method that is efficient and accurate for a highly heterogeneous velocity structure is applied to calculate p and s wave traveltimes from the source to receiving stations all earthquakes in the taiwan central weather bureau s catalogue are then relocated using the resultant 3 d vp and vs models the depth of the moho varies significantly especially along the east west direction in the western coastal plain and western foothills the depth of the moho is around 35 km which deepens gradually eastward reaches a maximum depth of 55 km beneath the eastern central mountain range shallows up rapidly beneath the longitudinal valley and coastal range and merges with the thin philippine sea plate offshore of eastern taiwan in central taiwan the central mountain range is bounded to the east and west by two steeply westward dipping active faults from the upper crust to a depth of about 30 km therefore the uplifted and thickened central mountain range serves as a backstop for the converging eurasian and philippine sea plates the crust beneath the central mountain range is characterized by a brittle high velocity and seismically active upper crust 15 km and a ductile low velocity and aseismic mid to lower crust below 15 km most probably due to the high geothermal activity from the excess heat supplied from the hot upper mantle beneath the thin oceanic crust to the east from the surrounding hotter upper mantle beneath the thickened continental crust and from shear heating during active collision the collision zone in eastern taiwan is characterized by an active and steeply eastward dipping seismic zone along a region of low vp and high vp vs ratio near the taitung region in southeastern taiwan it transforms into an active westward steeply dipping seismic zone along a transition zone between the high vp and vs oceanic crust and the low vp and vs continental crust near hualien region in central eastern taiwan there is no apparent seismicity within many sedimentary basins imaged from the tomographic inversion however af ew basins are either bounded on one side by an active fault or are characterized by blind faults beneath the geometry of the subduction zone in northeastern taiwan can be clearly imaged from the relocated earthquake locations behind the subduction a region of low vp and high vp vs ratio at depths of 5 to 10 km can be identified beneath the tatun chilung volcano group indicating a potential magma reservoir two steeply dipping linear seismic zones in the volcano region may mark the upward escape paths of the magmatic materials in the region

sonic anemometers in aeolian sediment transport research abstract fast response wind and turbulence instruments including sonic anemometers are used more and more in aeolian sediment transport research these instruments give information on mean wind but also on fluctuations and turbulent statistics such as the uw covariance which is a direct measure of reynolds stress rs and friction velocity this paper discusses the interpretation of sonic anemometer data the transformations needed to get proper results and turbulence spectra and how they are influenced by instrument size sampling frequency and measurement height turbulence spectra characterize how much the different frequencies in the turbulent signals contribute to the variance of wind speed or to the covariance of horizontal and vertical wind speed they are important in determining the measurement strategy when working with fast response instruments such as sonic anemometers and are useful for interpreting the measurement results choices on the type of sonic anemometer observation height sampling period sampling frequency and filtering can be made on the basis of expected high and low frequency losses in turbulent signals which are affected by those variables as well as wind speed and atmospheric stability friction velocity and rs important variables in aeolian sediment transport research are very sensitive to tilt or slope errors during a field experiment the slope sensitivity of the rs was established as 9 per degree of slope which is 1 5 times the value reported in literature on the basis of theoretical considerations an important reason for the difference probably is the large influence of streamline curvature on turbulence statistics and thereby on the slope sensitivity of the rs an error of 9 per degree of slope in the rs will translate into an error of approximately 4 per degree of slope in the calculated friction velocity space time correlation of the horizontal wind speed is much larger than that of the vertical wind speed and the instantaneous rs this largely explains why in previous studies a poor correlation was found between instantaneous rs measured at 3 m height and saltation flux near the surface whereas the correlation between wind speed at some height and saltation flux was much better therefore the poor correlation between rs away from the surface and saltation flux does not contradict that saltation flux is caused by rs

joint vp and vs tomography of taiwan implications for subduction collision orogeny abstract the taiwan orogen sitting at the pre rifted eurasian margin and bracketed by two sub orthogonal subduction systems is subject to complicated geotectonics motivating numerous seismic tomographic studies in local and regional scale most of them obtained p wave velocities vp or with vp vs ratio jointly but few were for s wave velocities vs with unprecedented amount of s wave data in this study a new set of well tuned vp vs and vp vs models were determined by an elaborate joint inversion scheme integrating the data of p and s wave travel times s p times and the borehole logging data for near surface correction into one system this allowed us to revisit the subduction collision process in detail by comparing vp vs and vp vs ratio simultaneously with enhanced slab imaging of philippine sea plate psp under northeast taiwan we are able to discriminate the plausible missing luzon forearc basement along the plate boundary and propose a skateboarding edge model in which the westernmost psp rides on the east subducted forearc basement and subducts northward underneath the eurasian lithosphere the underlain forearc basement along with the psp subduction then results in the anomalous shallow double seismic zone observed only near taiwan island meanwhile at around 121 e the north subducting psp seems dipping west to collide with the deep eurasian lithosphere shaping an irregular corner wedge structure in addition we evaluate our model with previous ones by waveform modeling approach and show the current performance of travel time tomography in taiwan region

detailed structures of the subducted philippine sea plate beneath northeast taiwan a new type of double seismic zone we studied the detailed structure of the subducted philippine sea plate beneath northeast taiwan where oblique subduction regional collision and back arc opening are all actively occurring simultaneous inversion for velocity structure and earthquake hypocenters are performed using the vast high quality data recorded by the taiwan seismic network we further supplement the inversion results with earthquake source parameters determined from inversion of teleseismic p and sh waveforms a critical step to define the position of plate interface and the state of strain within the subducted slab the most interesting feature is that relocated hypocenters tend to occur along a two layered structure the upper layer is located immediately below the plate interface and extends down to 70 80 km at a dip of 40 50 below approximately 100 km the dip increases dramatically to 70 80 the lower layer commences at 45 50 km and stays approximately parallel to the upper layer with a separation of 15 5 km in between down to 70 80 km below that the separation decreases and the two layers seem to gradually merge into one wadati benioff zone we propose to term the classic double seismic zones observed beneath japan and kuril as type i and that we observed as type ii respectively a global survey indicates that type ii double seismic zones are also observed in new zealand near the southernmost north island cascadia just north of the mendocino triple junction and the cook inlet area of alaska all of them are located near the termini of subducted slabs in a tectonic setting of oblique subduction we interpret the seismogenesis of type ii double seismic zones as reflecting the lateral compressive stress between the subducted plate and the adjacent lithosphere originating from oblique subduction and the downdip extension from slab pulling force the upper seismic layer represents seismicity occurring in the upper crust of a subducted plate and or along the plate interface whereas the lower layer is associated with events in the uppermost mantle

crust mantle boundaries in the taiwan luzon arc continent collision system determined from local earthquake tomography and 1d models implications for the mode of subduction polarity reversal abstract in order to better understand the mode of subduction polarity reversal in the taiwan luzon arc continent collision zone we mapped its crust mantle boundaries using local earthquake tomography by contouring surfaces of constant vp 7 5 km s 1 we identified three moho discontinuities and the plate interface that juxtaposes eurasian lower crust against mantle lithosphere of the philippine sea plate the plate interface dips to the east under southeastern taiwan and steepens progressively towards north until it becomes vertical at 23 7 n from there it continues northward in a vertical orientation until the limit of the tomographic model inhibited further mapping for the moho additional depth constraints were derived from 1d models using p wave arrivals of local earthquakes the mohos of the eurasian and philippine sea plates are disconnected across the plate interface beneath southern taiwan the eurasian moho dips to the east at 50 60 following the orientation of the plate boundary and continuous with the benioff zone towards north the eurasian moho steepens into subvertical again together with the plate boundary the philippine sea plate moho exhibits a synform like crustal root interpreted as the base of the magmatic luzon arc towards the north this root deepens from 30 to 70 km underneath the ryukyu trench in northernmost taiwan the hinge of the vertically subducting eurasian slab steps westward out of the thrust belt leaving the deformation front to the east inactive and giving way to the north dipping philippine sea plate a subhorizontal moho at 30 35 km depth overlies the north dipping philippine sea slab and is interpreted as a newly formed moho established after westward rollback and delamination of the subducting eurasian slab in combination these data support a model of progressive subduction polarity reversal in which a tear within the eurasian lithosphere propagated southwestward deactivating the deformation front

lithospheric structure of taiwan from gravity modelling and sequential inversion of seismological and gravity data abstract using a bouguer anomaly map and a dense seismic data set we have performed two studies in order to improve our knowledge of the deep structure of taiwan first we model the bouguer anomaly along a profile crossing the island using simple forward modelling the modelling is 2d with the hypothesis of cylindrical symmetry second we present a joint analysis of gravity anomaly and seismic arrival time data recorded in taiwan an initial velocity model has been obtained by local earthquake tomography let of the seismological data the let velocity model was used to construct an initial 3d gravity model using a linear velocity density relationship birch s law the synthetic bouguer anomaly calculated for this model has the same shape and wavelength as the observed anomaly however some characteristics of the anomaly map are not retrieved to derive a crustal velocity density model which accounts for both types of observations we performed a sequential inversion of seismological and gravity data the variance reduction of the arrival time data for the final sequential model was comparable to the variance reduction obtained by simple let moreover the sequential model explained about 80 of the observed gravity anomaly new 3d model of taiwan lithosphere is presented

post collisional magmatism around northern taiwan and its relation with opening of the okinawa trough being part of an active mountain belt formed by oblique collision of the luzon arc with asia northern taiwan and the offshore islets are marked by a series of latest pliocene quaternary volcanoes whose eruptions have been conventionally ascribed to westward propagation of the ryukyu volcanic arc on the basis of new geochemical data along with supporting geologic and geophysical evidence we propose instead that this young volcanism resulted from post collisional lithospheric extension in the northern taiwan mountain belt and the mantle source regions involved in the melt generation have been significantly modified by the nearby ryukyu subduction related processes magmas thus produced through the northeast to the southwest in the northern taiwan volcanic zone ntvz vary from low k to calc alkaline and then shoshonitic compositions such a spatial geochemical variation characterized by southwestward increase in enrichments of potassium and incompatible trace elements runs subparallel to the southwestern part of the present day ryukyu trench the geochemical variation that can be explained by southwestward decrease in degrees of partial melting of the mantle sources is reconciled with a southwestward weakening extensional regime observed in the ntvz the post collisional extension in northern taiwan furthermore might have played a role in reactivation of the opening in the middle okinawa trough and gave way to its rapid southwestward propagation with associated development of the ryukyu subduction zone west of124oe this nascent subduction in turn resulted in abundant submarine volcanoes which delineate an embryo volcanic front along the southern margin of the trough therefore the southwestern okinawa trough is not a fore arc basin as had been previously alleged but represents an atypical back arc basin which developed broadly synchronously or prior to its arc trench system in this particular collision extension subduction tectonic environment 1999 elsevier science b v all rights reserved

toward real time regional earthquake simulation ii real time online earthquake simulation ros of taiwan earthquakes abstract we developed a real time online earthquake simulation system ros to simulate regional earthquakes in taiwan the ros uses a centroid moment tensor solution of seismic events from a real time moment tensor monitoring system rmt which provides all the point source parameters including the event origin time hypocentral location moment magnitude and focal mechanism within 2 min after the occurrence of an earthquake then all of the source parameters are automatically forwarded to the ros to perform an earthquake simulation which is based on a spectral element method sem a new island wide high resolution sem mesh model is developed for the whole taiwan in this study we have improved sem mesh quality by introducing a thin high resolution mesh layer near the surface to accommodate steep and rapidly varying topography the mesh for the shallow sedimentary basin is adjusted to reflect its complex geometry and sharp lateral velocity contrasts the grid resolution at the surface is about 545 m which is sufficient to resolve topography and tomography data for simulations accurate up to 1 0 hz the ros is also an infrastructural service making online earthquake simulation feasible users can conduct their own earthquake simulation by providing a set of source parameters through the ros webpage for visualization a shakemovie and shakemap are produced during the simulation the time needed for one event is roughly 3 min for a 70 s ground motion simulation the ros is operated online at the institute of earth sciences academia sinica http ros earth sinica edu tw our long term goal for the ros system is to contribute to public earth science outreach and to realize seismic ground motion prediction in real time

the effect of turbulent flow structures on saltation sand transport in the atmospheric boundary layer the effect of turbulent flow structures on saltation sand transport was studied during two convective storms in niger west africa continuous synchronous measurements of saltation fluxes and turbulent velocity fluctuations were made with a sampling frequency of 1 hz the shear stress production was determined from the vertical and streamwise velocity fluctuations the greatest stress bearing events were classified as turbulent structures with sweep ejection inward interaction and outward interaction described according to the quadrant technique the classified turbulent structures accounted for 63 5 per cent of the average shear stress during the first storm and 56 0 per cent during the second storm the percentage of active time was only 20 6 per cent and 15 8 per cent respectively high saltation fluxes were associated with sweeps and outward interactions these two structures contribute positively sweeps and negatively outward interactions to the shear stress but have in common that the streamwise velocity component is higher than average therefore the horizontal drag force seems primarily responsible for saltation sand transport and not the shear stress this was also reflected by the low correlation coefficients r between shear stress and saltation flux 0 12 and 0 14 respectively while the correlation coefficients between the streamwise velocity component and saltation flux were much higher 0 65 and 0 57 respectively

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

foredune accretion under offshore winds experiments carried out at magilligan strand on the north coast of ireland suggest that topographic steering of offshore winds is an important facet of the aeolian sediment transport system at this location a five day study 18 22 june 2005 investigated the pattern of airflow over the foredune while simultaneously collecting data on sediment flux a simple instrument setup was used to characterise the airflow a sonic anemometer was placed at the dune crest with another on the mid beach horizontal traps electronic and integrating measured sediment flux on the supra tidal beach results show that offshore wind flow deviated from its original direction in the lee of the dune seaward of foredune the change in direction was not a simple steering of flow to a singular new direction but rather flow separation resulting in turbulent multidirectional flow including reversal traps located at the foot of the dune scarp recorded sediment transport during events forced by offshore winds data from sediment traps wind and observational evidence have been used to argue that sedimentation was occurring in this zone when budgeting for sediment movement within beach dune systems it is important to take into account the effect of secondary airflow patterns during offshore winds and their role in constructive processes of foredune formation particularly in post storm recovery c 2008 elsevier b v all rights reserved

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

initiation of arc magmatism in an embryonic continental rifting zone of the southernmost part of okinawa trough the okinawa trough is a young intracontinental backarc basin that has formed behind the ryukyu arc trench system since late miocene time in the southernmost part of the okinawa trough spot a cluster of active submarine volcanoes delineates a volcanic belt which is located only 100 km above the wadati benioff zone we report herein new major and trace element data for the spot volcanic rocks these rocks show a compositional range from medium k andesite to rhyolite their geochemical characteristics are similar to those of pre backarc rifting volcanic rocks from the central ryukyu arc and different from those of backarc basin lavas from the middle okinawa trough and the post backarc rifting ryukyu arc volcanics therefore despite being topographically contiguous with the rest of the trough the spot that developed in the quaternary is not a simple backarc basin but instead an embryonic rift zone in which early arc volcanism occurs as a result of the ryukyu subduction

wind forces and related saltation transport the effect of several wind characteristics on sand transport was studied in three experiments in north burkina faso west africa the first experiment is used to analyse the relation between wind speed and shear stress fluctuations across height the second experiment is used to study the relation of these wind characteristics with saltation transport for fourteen convective storms registered during the rainy seasons of 2002 and 2003 the effect of sampling time is studied for two of these convective storms the third experiment relates the turbulent structures of four convective storms to saltation transport wind speed measurements were undertaken with two sonic anemometers and sediment transport was measured by two saltiphones the sampling frequency was either 8 or 16 hz the sonic frame of reference was rotated according to a triple rotation horizontal fluctuations showed a fairly good correlation with height because the wind speed at both sensors was affected by the same vortices the correlation coefficients ranged from 0 42 when the distance between the sensors was 1 75 m to 0 92 when the distance was 0 25 m the instantaneous reynolds stress had the weakest correlation correlation coefficient of 0 05 at 1 75 m between the sensors and 0 56 at 0 25 m between the sensors because the momentum at 2 m above the soil surface is transported by different eddies than those close to the ground this also explains the fairly good correlation coefficients between the horizontal components of the wind and saltation compared to the poor correlations between instantaneous reynolds stress and saltation an increase in sampling time did not have much impact on these correlation coefficients up to sampling periods of about 30 s however this sampling interval would be too coarse to describe the vertical wind component adequately the classification of the moments of shear stress into the turbulent structures sweeps ejections inward and outward interactions showed that the mean saltation flux is higher at sweeps and outward interactions than at ejections and inward interactions also saltation occurred more often during sweeps and outward interactions than during ejections and inward interactions

turbulent reynolds stress and quadrant event activity in wind flow over a coastal foredune recent research on quasi instantaneous turbulent kinematic reynolds stresses rs u w and decomposed quadrant event activity e g ejections and sweeps over dunes in fluvial settings and in wind tunnels has shown that turbulent stresses at the toe of a dune often exceed time averaged streamwise shear stress u 2 estimates it is believed that semi coherent turbulent structures are conveyed toward the bed along concave streamlines in this region and that impact of these structures cause fluctuations in local surface stresses that assist in grain entrainment this has been hypothesized to explain how sand is supplied to the windward slope through a region of flow stagnation toward the crest surface stress increases and becomes dominated by streamwise accelerations resulting from streamline compression and convexity that suppress vertical motions high frequency 32 hz measurements of turbulent wind flow from 3 d ultrasonic anemometers are analyzed for oblique onshore flow over a vegetated coastal foredune in prince edward island canada reynolds stress and quadrant activity distributions varied with height 0 60 m and 1 66 m and location over the dune in general quadrant 2 ejection u 0 and quadrant 4 sweep activity u 0 w 0 w 0 and quadrant 3 inward interaction u 0 w 0 activity on the lower stoss slope significant ejection and sweep event activity was most frequent 85 to 92 ejections plus sweeps whereas at the upper crest significant ejection and sweep activity became less frequent while significant outward and inward interactions increased in frequency 25 to 36 an exuberance effect i e changing shape of quadrant frequency distribution skewed toward ejection and sweep activity is observed whereby streamline compression and convexity effects inhibit vertical fluctuations in flow and thus reduce the frequency of ejections and sweep activity toward the crest in separated flow in the lee of the crest quadrant distributions were more symmetrical as a result of more mixed multi directional flow these trends in turbulent event distributions and reynolds stress have implications for sediment transport dynamics across the dune and may help to explain sand transport potential and dune maintenance for example areas with a high frequency of ejection and sweep activity may have higher rates of sediment entrainment and transport whereas areas with lower ejection and sweep activity and an increase in outward and inward interactions which contribute negatively to reynolds stress generation may experience a greater potential for deposition further research on associations between quadrant event activity and coincident sand transport is required to confirm this hypothesis and the resultant significance of the flow exuberance effect in aeolian dune morphodynamics

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

timescale dependence of aeolian sand flux observations under atmospheric turbulence 1 the transport of sand in saltation is driven by the persistently unsteady stresses exerted by turbulent winds based on coupled high frequency observations of wind velocity and sand flux on a desert dune during intermittent saltation we show here how observations of saltation by natural winds depend significantly on the timescale and method used for determining shear stress and sand flux the correlation between sand flux and excess shear stress stress above a threshold value systematically improves for longer averaging timescale t and is better for stress determined by the law of the wall versus the reynolds stress method fitting parameters for the stress flux relationship do not converge with increasing t which may be explained by the nonstationary nature of wind velocity statistics we show how it may be possible based on the scale dependent statistics of stress fluctuations to rescale saltation flux predictions for wind observations made at different timescales however our observations indicate hysteresis and time lags in thresholds for initiation and cessation of saltation which complicate threshold based approaches to predicting sediment transport at different timescales

sand transportation and reverse patterns over leeward face of sand dune abstract sand saltation has complex interactions with turbulent flow and dune form most models of wind blown sand consider ideal circumstances such as steady wind velocity and a flat surface and the bulk of data on wind flow and sand transport over an individual dune has focused mostly on the influence of dune shape or inter dune space on the wind flow neglecting the effect of morphology on sand saltation particularly airflow and sand transportation over the leeward slope wind flow structures over the leeward slope of sand dunes have a fundamental influence on the organization of sand dunes in order to understand sand dune dynamics lee face airflow and sediment transportation should be paid more attention previous field observations could not measure turbulent flow structure well because of the limited observation points and the influence of experiment structure on wind field in addition the reverse sand particles over leeward face could not be collected by sand trap in field numerous field observations could not measure turbulent flow structure because of the limited observation points and the influence of experimental structures on the wind field in addition the reverse transport of sand particles over leeward face could not be collected by sand traps in field therefore this paper aims to investigate the turbulent flow structure and sand transport pattern over the leeward slope a numerical model of sand saltation over slope terrain is constructed which also considers the coupling effects between air flow and sand particles the large eddy simulation method is used to model turbulent flow sand transport is simulated by tracking the trajectory of each sand particle the results show that terrain significantly alters the turbulent air flow structure and wind blown sand movement especially over the leeward slope here mass flux increases initially and then decreases with height in the reversed flow region in the direction of wind flow and the mass flux decreases with height in the reversed direction the height of 0 5 h is the height of vortex core in the reversed flow region the vortex core is a critical point in the flow region where few particles are transited in the reversed region the reversed mass flux of sand particles is 25 of the mass flux in the flow direction this research may contribute to scientific understanding of the mechanisms of sand motion and wind flow over leeward of dune and it is likely to be significant in desertification control

origin of the southern okinawa trough volcanism from detailed seismic tomography 1 magmatism associated with subducting plate edges or slab tears has been suggested in the southern okinawa trough the cross back arc volcanic trail which consists of a cluster of about 70 seamounts is located above a ryukyu slab tear lying along the 123 3 e meridian in november 2003 more than 3300 earthquakes recorded in this area by 15 ocean bottom seismometers and surrounding land stations during a period of 12 days were used to determine the three dimensional vp and vs velocity structures and vp vs ratios a mantle inflow characterized by low vp and vs and high vp vs passing through the slab tear is imaged the fluid and or melt component is rising obliquely from the slab tear in the directions of the cross back arc volcanic trail the northern slope of the southern okinawa trough and to north of iriomote island the asthenospheric intake is also imaged by an inclined chip like high vp vs and low vp and vs body dipping northerly which might be linked to the slab retreat west of the slab tear most of the earthquakes are located around low vp and vs and high vp vs bodies which suggests that the seismicity is related to magmatic and or fluid activities east of it earthquakes are concentrated in an area characterized by high vp and vs velocities and low vp vs suggesting that the magma chamber is absent beneath the axial part of the trough and that normal faulting is the main factor controlling the seismicity

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

investigating the lithospheric velocity structures beneath the taiwan region by nonlinear joint inversion of local and teleseismic p wave data slab continuity and deflection the interaction between two flipping subduction systems shapes the complicated lithospheric structures and dynamics around the taiwan region whether and in what form the eurasian plate subducts deforms under taiwan island is critical to the debate of tectonic models although an east dipping high velocity anomaly down to a depth below 200 km has been reported previously its detailed morphology remains uncertain and leads to different interpretations with a two step strategy of nonlinear joint inversion the slab images of the eurasian plate were retrieved in a geometry that is hyperthin in the south becoming massive and steeper in the central and severely deformed in the north the possible depth and dimension of a slab break were also investigated through synthetic tests of whether the slab had torn moreover the slab deflection found at 23 2 n latitude seems to correspond to where the nonvolcanic tremors and recent nw se striking structures have occurred in southern taiwan

field measurements of mean and turbulent airflow over a barchan sand dune advances in our knowledge of the aeolian processes governing sand dune dynamics have been restricted by a reliance on measures of time averaged airflow such as shear velocity u it has become clear that such measures are incapable of explaining the complete dynamics of sediment transport across dune surfaces past evidence from wind tunnel and modelling studies has suggested that in some regions on a dune s surface the sediment transport might be better explained through investigations of the turbulent nature of the airflow however to date there have been no field studies providing data on the turbulent characteristics of the airflow around dunes with which to support or refute such hypotheses the field investigation presented here provides mean and turbulent airflow measurements across the centre line of a barchan sand dune in namibia data were collected using arrays of sonic anemometers and were compared with sand flux data measured using wedge shaped traps results support previously published data derived from wind tunnels and numerical models the decline in mean wind velocity at the upwind toe of the dune is shown to coincide with a rise in turbulence whilst mean velocity acceleration on the upper slope corresponds with a general decline in measured turbulence analysis of the components of reynold shear stress u w and normal stresses u 2 and w 2 supports the notion that the development of flow turbulence along the dune centre line is likely to be associated with the interplay between streamline curvature and mean flow deceleration acceleration it is suggested that due to the nature of its calculation turbulence intensity is a measure of less practical use than direct assessments of the individual components of reynolds stress particularly the instantaneous horizontal streamwise component u 2 and shear stress u w whilst increases in reynolds shear stress and the horizontal streamwise component of stress in the toe region of the dune may effectively explain the maintenance of sand flux in a region of declining mean velocity they have much less explanatory power for sand flux on the upper windward slope and in the crestal region of the dune here it is suggested that mean flow acceleration is likely to provide the most significant driving force on sand flux possibly augmented by a rise in the horizontal streamwise component of reynolds stress u 2 in the crest brink region therefore although wind turbulence is considered to be of fundamental importance in explaining the sediment transport dynamics across the dune s surface it is recognised that the interaction between mean flow deceleration acceleration streamline curvature and individual components of reynolds stress is complex and the identification of a single element of flow that offers a panacea for accounting for sand flux and dune dynamics is difficult to find

temporal and spatial variation of stress field in taiwan from 1991 to 2007 insights from comprehensive first motion focal mechanism catalog article i nfo the total amount of 4 761 focal mechanisms was determined based on p wave first motion polarities from 1991 to 2007 in taiwan region this dataset offers us a good opportunity to examine temporal and spatial variability of the stress field we find that the spatial variations of stress axes are mainly controlled by tectonic structures while the temporal changes are greatly influenced by the chi chi earthquake the orientation of the maximum horizontal compressive stress axes sh shows a general agreement with the direction of plate motion between a depth range of 0 30 km the 20 anticlockwise rotation of sh from the longitudinal valley lv to western taiwan is probably caused by the left lateral motion on the longitudinal valley fault lvf that has consumed part of the oblique motion of plate convergence on the other hand part of the oblique convergence is transferred into the central range and the hsuehshan range judging from counterclockwise rotation of sh from east to west and strike slip faulting in the hsuehshan range most events with a depth greater than 30 km occur offshore eastern taiwan and the azimuth of sh is close to e w directed different from nw se directed at shallow depths this may infer the existence of the transition of lithosphere rheology in offshore eastern taiwan the trends of sh in the depth of 0 10 km are strongly affected by the coseismic stress change of the chi chi earthquake in the northern half of the chi chi rupture area the trends of sh rotate 30 clockwise and the stress ratio increased by a factor of six after the mainshock the orientations of sh still differ by 30 in 2007 comparing to that in the period before the chi chi earthquake the variation of sh trend is more diverse in the southern half of the rupture area showing 20 counterclockwise rotation immediately after the 1999 mainshock followed by a clockwise rotation the trend of sh returns to the pre seismic direction of 110 in 2001 these notable changes of sh before and after the chi chi mainshock suggests that the magnitude of background stress in the rupture area is close to the coseismic stress drop we also recognize a significant 10 counterclockwise rotation of sh in the entire chi chi rupture area between 1991 and 1999 before the earthquake took place however to the south of the chi chi rupture the trends of sh remain little changed before and after the chi chi earthquake

seismic tomography of taiwan improved constraints from a dense network of strong motion stations in this study a large collection of 41 141 s p times from the untapped records of the taiwan strong motion instrumentation program tsmip network is combined with the p and s wave arrival times from the taiwan central weather bureau seismic network cwbsn to image the v p and v p v s structures beneath taiwan the records from the 680 tsmip stations throughout taiwan in the past 15 years enhance the path coverage and the resolution in the tomography inversions tremendously our result for the v p structure largely confirms previous studies but brings better constraint on the v p v s structure the colliding luzon volcanic arc is characterized by a belt of high v p and high v p v s with high seismicity that includes the offshore islands of lutao and lanyu and the coastal ranges in eastern taiwan at the depth between about 13 and 25 km this high v p v s belt can be traced to the subduction zone in the region between hualian and ilan in the deeper portion the shallow portions of the southwestern coastal plain and the pingtung region are also characterized by a belt of high v p v s with lower seismicity most of the events occurred at the base of the high v p v s zones we suggest that material strength in those regions may be too low to accumulate stress which may indicate water saturated young sediments finally the central range region is characterized by a low v p v s belt

streamline correction for the analysis of boundary layer turbulence abstract improvements in the design and affordability of ultrasonic anemometers have provided significant contributions to aeolian research by facilitating high frequency monitoring of three dimensional wind velocities from these data it is possible to calculate quasi instantaneous reynolds stresses to evaluate boundary layer turbulence moving beyond time averaged measures such as shear velocity u as ultrasonic anemometry is used more frequently in aeolian geomorphology it is important to question accepted conventions concerning data processing and analysis this paper examines data processing questions associated with the application of ultrasonic anemometry to field studies in aeolian geomorphology through an investigation of three streamline correction routines the two step three step and planar fit methods on data recorded on a gently sloping beach at magilligan strand northern ireland in may 2010 the planar fit technique has not previously been used in aeolian geomorphology results are compared with data that have been corrected only for wind direction yaw the effects that these different methods have on quadrant analysis and reynolds stress calculation are discussed streamline correction is applied as a time variable procedure using a characteristic timescale of 8 s following analysis of the resultant wind speed energy spectrum it is found that reynolds shear stress is dependent on streamline correction method with run mean estimates of resultant horizontal shear stress ranging from 0 05 to 0 11 n m 2 depending on the technique the two step method consistently maximises the shear stress and when the resultant horizontal shear is calculated it produces the most robust estimate for application to aeolian research in contrast the different methods have little effect on the identification or sequencing of turbulent structures using quadrant analysis streamline correction is an essential processing step when using reynolds decomposition however the method used has an effect on data quality as each correction re distributes stress across the whole tensor altering both normal and shear stresses the total stress tensor magnitude itself is a conserved value and may provide a suitable parameter to quantify boundary layer turbulence

okinawa trough backarc basin early tectonic and magmatic evolution the okinawa trough lying between japan and taiwan is a backarc basin formed by extension within the continental lithosphere behind the ryukyu trench arc system stress directions associated with the two last extensional phases in the southwestern okinawa trough have been deduced from a comparison with analog modeling the direction of extension is n 150 o for the pleistocene phase of extension 2 0 1 ma and n 170 o for the late pleistocene holocene phase of extension 0 1 0 ma the present day ryukyu volcanic arc a narrow continuous feature extending from japan to taiwan is located on the eastern side of the okinawa trough 80 100 km above the wadati benioff zone the minimum depth for emplacement of arc magmatism scarce present day backarc volcanism appears in the middle and southern okinawa trough within linear en echelon bathymetric depressions a n045 o oriented seamount volcanic chain cuts across obliquely the southwestern okinawa trough and lies in the direct line of extension of the gagua ridge a n s linear volcanic feature of the philippine sea plate associated with this extension of the gagua ridge a large reentrant located at the base of the ryukyu prism the uplift of part of the nanao forearc basin and the deformation of the sedimentary arc suggest that the voluminous cross backarc volcanism could be tied to the subduction of the gagua ridge located there at a depth of 80 100 km beneath the backarc basin a second area of anomalous volcanism has been identified in the middle okinawa trough in the ene extension of the daito ridge a wnw ese 400 km long volcanic feature of the philippine sea plate we suggest that the gagua and daito ridges initially induced stress at the base of the arc which is still brittle and cracks propagated through the overlying brittle lithosphere allowing magmas with arc affinities to erupt at the seafloor this excessive magmatism reaches the seafloor through conduits which preferentially follow in their shallowest portion the crustal normal faults of the backarc rifts the okinawa trough is consequently still in an early stage of evolving from arc type to backarc activity

three dimensional p and s wave velocity structures beneath taiwan deep structure beneath an arc continent collision the island of taiwan is a byproduct of one of the few active collisions between a continent represented by the eurasian continental shelf and an island arc represented by the northern extension of the luzon arc on the philippine sea plate to understand better the evolution and current tectonics of this collision we selected 1260 well recorded earthquakes from an initial set of 50 000 located by the taiwan telemetered seismograph network for the determination of one and three dimensional p and s wave velocity structures beneath the island the results for both structures and earthquake relocations reveal that the island is divided tectonically into three distinct zones in the east the philippine oceanic plate is underthrusting the eurasian plate east of a north south boundary that is well defined by both seismic activity and a region of high velocity in the south the eurasian continental plate is underthrusting the philippine sea plate south of an east west boundary at about 23 n that is sharply defined by both subcrustal seismicity and a zone of relatively low velocities the dip of the subducted continent is shallow until it reaches the luzon island arc 50 km east of the main island structure under the main part of the island north of 230n reveals a shallow dipping zone of low velocities in the west above 25 km depth that narrows and steepens below that depth to at least 50 km beneath the central range the dip of this low velocity region is outlined by a narrow zone of seismicity that extends to depths of 100 km this seismic zone lies in a velocity saddle and marks an apparent offset in the central low velocity region at 24 n evidence for the subduction of the eurasian continent beneath taiwan therefore exists everywhere beneath the island and the low velocity regions in the mantle support but do not require the subduction of some 6 16 km of lower continental crust to depths of at least 50 km the distinct change in both the velocity structure and the seismic activity at 23 n and the offset of the low velocity regions at 24 n argue for abrupt changes in the nature of subduction across these latitudes these changes may be caused by an interaction of the luzon island arc with the subducted continental shelf that mimics a similar interaction evident at the surface finally the velocity structure of the subducting philippine sea plate suggests that the earthquakes beneath 70 km depth are not occurring within the highest velocity regions of the plate but are probably located near the upper edge of the plate there is also evidence from earthquake locations and velocity structure that suggests that the subducting plate is segmented and that subduction is presently occurring as far south as 22 n

relocation of the 2006 pingtung earthquake sequence and seismotectonics in southern taiwan article i nfo we combine the arrival time data from eleven ocean bottom seismometers obs and from permanent stations of the central weather bureau seismic network cwbsn and the taiwan strong motion instrumentation program tsmip to improve the three dimensional vp and vp vs structures in the taiwan region relocate hypocenters of earthquakes in southern taiwan including the 2006 pingtung earthquake sequence and determine the focal mechanisms of some of the larger events our tomography result suggests that in the region south of taiwan the eurasian continental lithosphere is being subducted beneath the oceanic philippine sea plate leading to a region of low p wave speed with seismicity on top of a relatively high p wave speed zone the hypocentral distribution of the pingtung earthquake sequence indicates that the first event in the initial two largest shocks was a normal faulting quake caused by the bending of the subducting lithosphere the second event perhaps triggered by the first one was a strike slip earthquake

high frequency sediment transport responses on a vegetated foredune wind flow and sand transport intensity were measured on the seaward slope of a vegetated foredune during a 16 h storm using an array of sonic anemometers and wenglor laser particle counters the foredune had a compound seaward slope with a wave cut scarp about 0 5 m high separating the upper vegetated portion from the lower dune ramp which was bare of vegetation wind direction veered from obliquely offshore at the start of the event to obliquely onshore during the storm peak and finally to directly onshore during the final 2 h as wind speed dropped to below threshold sand transport was initially inhibited by a brief period of rain at the start of the event but as the surface dried and wind speed increased sand transport was initiated over the entire seaward slope transport intensity was quite variable both temporally and spatially on the upper slope as a result of fluctuating wind speed and direction but overall magnitudes were similar over the whole length ten minute average transport intensity correlates strongly with mean wind speed measured at the dune crest and there is also strong correlation between instantaneous wind speed and transport intensity measured at the same locations when the data are smoothed with a 10 s running mean transport on the beach for onshore winds is decoupled from that on the seaward slope above the small scarp when the wind angle is highly oblique but for wind angles 45 from shore perpendicular some sand is transported onto the lower slope copyright 2012 john wiley sons ltd

a tear fault boundary between the taiwan orogen and the ryukyu subduction zone more than twenty seismic lines were analysed across the intersection of the taiwan orogen and the ryukyu arc forearc system the main results of this study concern the recognition of two superposed sedimentary basins that were previously unknown we have named the recent one the hoping basin and the older tectonized one the suao basin the suao basin containing sediments more than 3 km thick records subsidence of about 3 km on its southwestern portion after being shortened in the direction of plate convergence on the basis of seismicity gravity present day plate kinematics as well as seismic reflection data we relate the dramatic subsidence of the suao basin to the initiation of a wnw trending tear fault within the philippine sea plate this tear fault decouples the subducting philippine sea plate to the northeast and the apparent overthrusting philippine sea plate in the coastal range to the southwest back arc rifting may thus occur in the westernmost section of the okinawa trough above the sinking philippine sea slab while lithospheric shortening prevails south of hualien as a consequence the triangular area located southwest of the tear fault between the taiwan coastline and the toe of the yaeyama ridge no longer belongs to the ryukyu forearc it is now passively transported with the non subducting portion of the philippine sea plate we propose an evolutionary model for the relative chronology of tectono sedimentary events in this region over the last 5 m y

melting features along the western ryukyu slab edge northeast taiwan tomographic evidence 1 behind the sedimentary ryukyu arc lies the okinawa trough whose termination is located at the tip of the ilan plain northern taiwan just above the ryukyu slab edge the present day active volcanic front is located 80 100 km above the ryukyu slab and extends from japan to kueishantao island an islet situated 10 km offshore the ilan plain between december 1990 and may 1999 3370 earthquakes recorded in northern taiwan by 65 seismic land stations were used to determine the three dimensional vp and vs velocity structures and vp vs ratios a low vs but high vp vs sausage like body 30 km in diameter lies within the eurasian mantle wedge on top of the western ryukyu slab extremity at depths ranging between 20 and 100 km we suggest that the h2o rich component formed by dehydration processes from subducting sediments oceanic crust and serpentinized mantle above the ryukyu slab and along the vertical portion of the slab edge might explain the presence of the sausage like body a low vs but high vp vs channel rises obliquely from the sausage like body at a depth of 40 km in direction of the andesitic kueishantao island we propose that the h2o rich component and or melt rise up from the sausage like body and interfere with the okinawa trough back arc basin magmas formed in the upper mantle lower crust then magmas propagate upward within the upper brittle crust through veins and or narrow conduits

boundary layer turbulence characteristics during aeolian saltation 1 a great deal of effort has been expended in measuring turbulence phenomena in clean air flows however no previous measurements have been successfully made of the vertical distributions of turbulence intensity and reynolds stress in a fully adjusted boundary layer flow saturated with saltating particles the present wind tunnel study addresses this knowledge gap using a custom designed laser doppler anemometer lda the amount of turbulence is found to increase with the introduction of saltating particles to the airflow over the lowest 15 of boundary layer vertical profiles of the streamwise wind speed provide friction velocities that lie well within the narrow range of those derived from direct measurement of the reynolds stress relative to clean air aeolian saltation is demonstrated to increase the magnitude but not the frequency of burst sweep events that primarily contribute to the total fluid stress within several millimeters above the bed surface all vertical profiles of wind speed converge upon a focal point as the local fluid stress declines toward the mobile bed

geomorphology of desert sand dunes a review of recent progress through the 1980s and 1990s studies of the geomorphology of desert sand dunes were dominated by field studies of wind flow and sand flow over individual dunes alongside these there were some attempts numerically to model dune development as well as some wind tunnel studies that investigated wind flow over dunes as developments with equipment allowed field measurements became more sophisticated however by the mid 1990s it was clear that even these more complex measurements were still unable to explain the mechanisms by which sand is entrained and transported most importantly the attempt to measure the stresses imposed by the wind on the sand surface proved impossible and the use of shear or friction velocity as a surrogate for shear stress also failed to deliver at the same time it has become apparent that turbulent structures in the flow may be as or more important in explaining sand flux in a development paralleled in fluvial geomorphology aeolian geomorphologists have attempted to measure and model turbulent structures over dunes progress has recently been made through the use of more complex numerical models based on computational fluid dynamics cfd some of the modelling work has also suggested that notions of dune equilibrium form may not be particularly helpful this range of recent developments has not meant that field studies are now redundant for linear dunes careful observations of individual dunes have provided important data about how the dunes develop but in this particular field some progress has been made through ground penetrating radar images of the internal structure of the dunes the paradigm for studies of desert dune geomorphology for several decades has been that good quality empirical data about wind flow and sand flux will enable us to understand how dunes are created and maintain their form at least some of the difficulty in the past arose from the plethora of undirected data generated by largely inductive field studies more recently attention has shifted although not completely to modelling approaches and very considerable progress has been made in developing models of dune development it is clear however that the models will continue to require accurate field observations in order for us to be able to develop a clear understanding of desert sand dune geomorphology

burst like sediment suspension events in a sand bed river in a much quoted paper jackson 1976 hypothesized that turbulent bursting motions such as those documented in laboratory boundary layers play a major role in alluvial sediment suspension to date the hypothesis remained largely untested due to difficulties in monitoring turbulent suspension in rivers this study provides field data documenting burst like turbulent motions over a sandy bed channel and quantifying the role of these motions in sand suspension the data were collected in a 10 m deep channel of the fraser river near mission british columbia canada turbulent fluctuations of both flow components downstream and normal to the bed along with the output of an optical suspended sediment sensor were monitored 1 m above the river bed typical flow velocities averaged 0 9 ms 1 at the sensors where mean suspended sediment concentrations were 500 mgl 1 decimetre height small dunes on the backs of larger half metre amplitude dunes covered the channel bed in the area brief but intense burst like ejection and inrush events were identified in the flow records where they are responsible for a high degree of intermittency in shear stress over the dunes 80 per cent of the turbulent momentum exchange across the 1 m level can be ascribed to such brief 3 8 s duration events active under 12 per cent of the time in addition the record of fluctuating sediment concentrations reveals these burst like motions to be highly effective in vertically mixing suspended sediment and thus ultimately in maintaining suspended sediment transport above the dune bed the bulk 60 and 90 per cent in two deployments of the vertical sediment mixing was accomplished by intense events active some 10 per cent of the time no discrete recurrence timescale for these burst like mixing events is evident however rather a continuous variation of return periods was observed as a function of the magnitude of vertical mixing event considered to that extent conceptual models of sediment transport in terms of burst events with a predictable recurrence such as proposed by jackson 1976 may be misleading

three dimensional velocity structure in northern california coast ranges from inversion of local earthquake arrival times abstract the three dimensional p wave velocity structure in the upper crust in the northern california coast ranges from santa rosa to ukiah is obtained by simultaneous inversion of local earthquake and refraction travel time data for velocity and hypocentral parameters in this region where there are lateral heterogeneities the three dimensional inversion provides important results not found in two dimensional refraction studies the medium is parameterized with velocities at the intersections gridpoints of a nonuniform three dimensional grid travel times are calculated with approximate ray tracing along arcuate paths which are reasonable for lateral distances less than 45 km inversions with 5 to 10 and 2 to 3 km gridspacing achieved 80 per cent decreases in data variance the inversion with 2 to 3 km gridspacing reduced the rms residual level to approximately the level of timing error anomalously low velocities to at least 6 km depth are found along the maacama and healdsburg rodgers creek hrc fault zones the low velocity region is widest at the en echelon junction of the maacama and hrc fault zones low velocities are associated with the clear lake basin where there is a thick sequence of volcanics interbedded with sedimentary deposits there is a high velocity body located southeast of the geysers between the maacama and collayomi fault zones which may be associated with igneous intrusives or high grade metamorphism related to an underlying inferred partial melt body modeled in previous teleseismic travel time studies there is some indication in the vapor dominated geothermal reservoir of lower velocities relative to surrounding water saturated rock as has been shown in some laboratory experiments hypocenter locations obtained in the inversion show little variation from hypocenters obtained through inversion for one dimensional velocity model and station corrections for the inversion with 2 to 3 km gridspacing however some groups of events become more tightly clustered

numerical simulation of aeolian dusty sand transport in a marginal desert region at the early entrainment stage abstract aeolian dusty sand transport in the marginal region of a desert is described numerically from first suspension motion at the early entrainment stage to the unsteady state within a moderate range 1000 m long 500 m high a two dimensional model is built for wind blown dust flow and the calculations are carried out using fluent software the simulation results describe an integrated picture of aeolian dusty sand transport including uplift suspension diffusion deposition and its space time concentration according to the features of a sand dust storm near the surface a volume concentration expression of ejection grains is developed as a boundary condition in the simulation approach the model is verified by comparing results with both experimental data from a wind tunnel and an analytic solution uniform dust sizes and r r distributions are used in the simulation gas solid two phase flow patterns with these grains are obtained in the downwind space including the turbulence intensity gas phase stream functions and solid volume concentration distributions the influence of wind velocities and grain sizes is analyzed from the simulation results spatial distributions of dust volume concentration in the early entrainment stage are described clearly different from coarse sands there is a clear band of uniformly saturated dust concentration in the region directly above the surface

modelling aeolian sand transport using a dynamic mass balancing approach abstract knowledge of the changing rate of sediment flux in space and time is essential for quantifying surface erosion and deposition in desert landscapes whilst many aeolian studies have relied on time averaged parameters such as wind velocity u and wind shear velocity u to determine sediment flux there is increasing field evidence that high frequency turbulence is an important driving force behind the entrainment and transport of sand at this scale of analysis inertia in the saltation system causes changes in sediment transport to lag behind de accelerations in flow however saltation inertia has yet to be incorporated into a functional sand transport model that can be used for predictive purposes in this study we present a new transport model that dynamically balances the sand mass being transported in the wind flow the dynamic mass balance dmb model we present accounts for high frequency variations in the horizontal u component of wind flow as saltation is most strongly associated with the positive u component of the wind the performance of the dmb model is tested by fitting it to two field derived namibia s skeleton coast datasets of wind velocity and sediment transport i a 10 min 10 hz measurement resolution dataset ii a 2 h 1 hz measurement resolution dataset the dmb model is shown to outperform two existing models that rely on time averaged wind velocity data e g radok 1977 dong et al 2003 when predicting sand transport over the two experiments for all measurement averaging intervals presented in this study 10 hz 10 min the dmb model predicted total saltation count to within at least 0 48 whereas the radok and dong models over or underestimated total count by up to 5 50 and 20 53 respectively the dmb model also produced more realistic less peaky time series of sand flux than the other two models and a more accurate distribution of sand flux data the best predictions of total sand transport are achieved using our dmb model at a temporal resolution of 4 s in cases where the temporal scale of investigation is relatively short on the order of minutes and at a resolution of 1 min for longer wind and transport datasets on the order of hours the proposed new sand transport model could prove to be significant for integrating turbulence scale transport processes into longer term macro scale landscape modelling of drylands

retraction numerical simulation of a wind sand flow under different conditions of atmospheric stability a theoretical model for wind sand flow is developed by considering the coupling between wind flow and sand particle motion the latter subject to the magnus effect under different atmospheric stability conditions using this model the characteristics of the wind sand flow are discussed in detail the results show that the atmospheric stability and the magnus effect both have a strong influence on wind profiles and on the trajectories of sand particles this approach produces results with characteristics that differ from those previously reported the latter only applying to atmospheric conditions of neutral stability the saltating sand reaches a greater height under non neutral stability than under neutral stability while the maximum horizontal distance is greater under unstable conditions and is smaller under stable conditions than under conditions of neutral stability

turbulent flow structures and aeolian sediment transport over a barchan sand dune 1 the turbulent structure of airflow over a barchan sand dune is determined using quadrant analysis of wind velocity data derived from sonic anemometers results indicate an increased frequency of ejection and sweep events in the toe region of the dune characteristic of the turbulent bursting process in contrast at the crest there was a significant increase in the occurrence of outward interactions combined with high frequency saltation data our analyses show that turbulent structures characterised by a positive streamwise fluctuating velocity u sweeps at the toe and outward interactions at the crest have a dominant influence on sand transport on the dune together accounting for up to 83 and 95 of transporting events at the toe and crest respectively

imaging source slip distribution by the back projection of p wave amplitudes from strong motion records a case study for the 2010 jiasian taiwan earthquake

spatial variation of seismogenic depths of crustal earthquakes in the taiwan region implications for seismic hazard assessment abstract this paper presents the first whole taiwan scale spatial variation of the seismogenic zone using a high quality crustal seismicity catalog the seismicity onset and cutoff depths i e seismogenic depths are determined by the earthquake depth moment distribution and used to define the upper and lower boundaries of the seismogenic zone respectively together with the published fault geometries and fault area moment magnitude relations the depth difference in the onset and cutoff depths i e seismogenic thickness is used as the fault width to determine the moment magnitudes of potential earthquakes for the major seismogenic faults results show that the largest mw7 9 8 0 potential earthquake may occur along the changhua fault in western taiwan where the seismic risk is relatively high and seismic hazard mitigation should be a matter of urgent concern in addition the first motion focal mechanism catalog is used to examine the relation between the seismogenic depths and earthquake source parameters for crustal earthquakes 50 km the shallowest onset and cutoff depths are observed for normal and strike slip events respectively this observation is different from the prediction of the conventional continental rheology model which states that thrust events have the shallowest cutoff depth thus a more sophisticated rheology model is necessary to explain our observed dependence of the seismogenic depths on faulting types meanwhile for intermediate to large crustal mw 4 depth 50 km earthquakes thrust events tend to occur at the bottom region of the seismogenic zone but normal and strike slip events distribute at a large depth range

sea bed noises reveal role of turbulent bursting phenomenon in sediment transport by tidal currents in many engineering and sedimentological applications it is usual to relate sediment movement to the time averaged bed shear stress however the shear stress exerted on the sea bed by tidal currents is intermittent in nature due to the turbulent bursting phenomenon1 2 sediment movement at the sea bed also occurs in bursts3 4 however attempts to link these two processes have failed because of the poor temporal resolution of existing sediment transport measurement techniques recent experimental advances now make this possible and we describe here the first detailed observations of sediment movement and fluid turbulence in tidal currents flowing over sandy gravels these results show that high stress values are a necessary but not sufficient condition for bed load sediment movement and that form drag on individual particles may have greater dynamical significance than the stress this work also suggests that it may now be possible to describe sediment movement at a level which is deterministic in terms of the turbulence and at very much shorter timescales than have been used previously

collisional tectonics between the eurasian and philippine sea plates from tomography evidences in southeast china abstract the upper mantle structure of southeast china is important for us to understand the deformation and mantle dynamics process associated with the interaction between the eurasian plate and philippine sea phs slab we determined a detailed three dimensional p wave velocity vp structure of the crust and upper mantle down to 400 km depth beneath southeast china by applying teleseismic tomography to 6869 high quality p wave arrival times the data were collected very carefully from the original seismograms of 635 teleseismic events recorded by 65 broadband stations deployed in southeast china our images show that the high vp phs slab subducts toward the north along the ryukyu trench at the latitude of about 24 n and extends down to 350 km depth and even more high vp anomalies are imaged in the upper mantle under central and southern taiwan which represent the subducted eurasian plate break off eurasian plate at a big angle subducting eastward is revealed under central taiwan at depths from the upper mantle to 400 km while continuous eurasian plate under south taiwan is mainly imaged from the moho down to 400 km depth a torn mantle window within the eurasian continent beneath central and northern taiwan created by the northward motion of the philippine sea plate is the upwelling path of the asthenosphere the tomographic images also show the low vp anomalies spread widely under the coastal areas of mainland china and taiwan strait the structure of the crust and upper mantle suggests that the mountain building process in the central part of taiwan is mainly attributed to the subduction collision tectonics at the boundary between the eurasian continental lithosphere and the subducting oceanic lithosphere of the phs slab

transition from oblique subduction to collision earthquakes in the southernmost ryukyu arc taiwan region tectonic characteristics of the region between taiwan and the southernmost ryukyu arc are inferred from a detailed analysis of local seismicity and source parameters of 62 recent earthquakes of 5 5 mb 6 6 five major seismogenic structures can be delineated the collision seismic zone csz the interface seismic zone isz the wadati benioff seismic zone wbsz the lateral compression seismic zone lcsz and the okinawa seismic zone osz in the csz located along the east coast of taiwan and offshore earthquake focal mechanisms show horizontal p axes distributed in two directions 287 10 and 333 16 possibly reflecting a strain partition associated with the relative plate convergence between the eurasia plate and the philippine sea plate the corresponding seismic strain tensor indicates a maximum compressive strain rate of 1 2 10 7 yr 1 along 293 and a comparable extension in vertical direction presumably resulted from plate collision in the region the geometry of the isz which is distorted significantly at its westernmost end can be approximated by a north dipping plane that is gradually pushed northward with increasing dip the seismogenic portion of the interface spans a short depth range from 10 km to 35 km a clear pattern of earthquake slip partition is observed the average slip vector residual is as large as 35 seismic strain patterns within the subducted philippine sea slab show predominantly downdip extension between 80 and 120 km and downdip compression at 270 km different from the pattern of strain segmentation observed for the rest of the ryukyu arc where the northern and southern portions are dominated by downdip extension and compression respectively owing to the large convergence obliquity the slab is descending at a rate significantly slower near taiwan than in the southern ryukyu thus we interpret the appearance of downdip extension within the subducted lithosphere as a combined result of oblique subduction and the slab s negative buoyancy a number of thrust or oblique strike slip earthquakes between isz and wbsz show a consistent pattern of lateral compression with p axes oriented roughly parallel to the local strike of the trench arc system they are probably due to the compressive strain originated from the collision and transmitted laterally within the lithosphere shallow normal faulting earthquakes show successive rotation of t axes from approximately n s in the okinawa trough to approximately e w in northeast taiwan possibly as a result of interaction between the extension from the opening of the okinawa trough and the compression from collision one normal event january 18 1991 mb 5 9 occurred in the central range with t axis roughly parallel to the structural trend of taiwan implying that the nature of the orogeny in taiwan has changed from thin skinned deformation to lithospheric collision involving the whole crust and uppermost mantle

the shuilikeng fault in the central taiwan mountain belt for over 200 km along strike the shuilikeng fault of taiwan separates miocene rocks of the western foothills from the largely eocene and oligocene rocks of the hsuehshan range to the east despite its importance in the taiwan mountain belt the structure and kinematics of the shuilikeng fault are not well known here we present results from new geological mapping along 100 km of its strike length at the surface the shuilikeng fault is a steeply east dipping brittle fault with a series of splays and bifurcations along its southern part it cuts an earlier fold and fault system outcrop kinematic data vary widely from thrusting to strike slip the surface data are integrated with a relocated and collapsed seismicity database to interpret the fault location at depth these data indicate that the shuilikeng fault can be traced to greater than 20 km depth some 260 focal mechanisms from this dataset indicate that its kinematics is overall transpressive from a regional perspective we interpret the shuilikeng fault to reactivate a pre existing rift related basin bounding fault to the east of which rocks in the hsuehshan range are being exhumed

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

aeolian sediment mass fluxes on a sandy soil in central patagonia abstract the climate of patagonia is semi arid and characterised by frequent strong winds wind erosion is potentially a serious soil degradation process that impacts long term sustainability of local agricultural systems but the conditions and the rates of wind erosion in this region have not been studied extensively the aim of this study was to quantify windblown mass transport on a sandy soil in central patagonia aeolian mass fluxes were measured in the valley of sarmiento chubut province argentina using two saltiphones and 24 modified wilson and cooke mwac sediment catchers the latter were installed along three transects 1 a control on a bare strip of land cleared of its natural vegetation to measure the maximum wind erosion 2 a similar transect protected by an artificial windbreak with an optical porosity of 50 and 3 a transect in a cherry orchard protected with the same type of windbreak nine windstorms were recorded throughout the experimental period storms with wind speed peaks of 20 m s 1 caused a total soil loss of 248 mg ha 1 in the control strip and heavily depleted the soil of its erodible fraction the artificial windbreak reduced the soil loss by 51 0 on average while no erosion was recorded in the cherry orchard measured maximum mass transport values were used to fit five sediment transport equations in order to select the best equation to integrate into a gis based wind erosion prediction system the kawamura 1964 equation showed the highest model efficiency and was considered to be the best sediment transport equation for the patagonia conditions it expresses total mass transport as a function of two empirical constants the threshold friction velocity u t and an erodibility coefficient cka it is concluded that wind erosion in central patagonia poses a serious risk of soil degradation once the natural vegetation is removed due to overgrazing or other anthropogenic activities

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

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

three dimensional seismic velocity structure of the san francisco bay area seismic travel times from the northern california earthquake catalogue and from the 1991 bay area seismic imaging experiment basix refraction survey were used to obtain a three dimensional model of the seismic velocity structure of the san francisco bay area nonlinear tomography was used to simultaneously invert for both velocity and hypocenters the new hypocenter inversion algorithm uses finite difference travel times and is an extension of an existing velocity tomography algorithm numerous inversions were performed with different parameters to test the reliability of the resulting velocity model most hypocenters were relocated 12 km under the sacramento river delta 6 km beneath livermore valley 5 km beneath the santa clara valley and 4 km beneath eastern san pablo bay the great valley sequence east of san francisco bay is 4 6 km thick a relatively high velocity body exists in the upper 10 km beneath the sonoma volcanic field but no evidence for a large intrusion or magma chamber exists in the crust under the geysers or the clear lake volcanic center lateral velocity contrasts indicate that the major strike slip faults extend sub vertically beneath their surface locations through most of the crust strong lateral velocity contrasts of 0 3 0 6 km s are observed across the san andreas fault in the middle crust and across the hayward rogers creek calaveras and greenville faults at shallow depth weaker velocity contrasts 0 1 0 3 km s exist across the san andreas hayward and rogers creek faults at all other depths low spatial resolution evidence in the lower crust suggests that the top of high velocity mafic rocks gets deeper from west to east and may be offset under the major faults the data suggest that the major strike slip faults extend sub vertically through the middle and perhaps the lower crust and juxtapose differing lithology due to accumulated strike slip motion the extent and physical properties of the major geologic units as constrained by the model should be used to improve studies of seismicity strong ground motion and regional stress