a numerical study of the long and short term temperature variability and thermal circulation in the north sea a three dimensional numerical study is presented of the seasonal semimonthly and tidal inertial cycles of temperature and density driven circulation within the north sea the simulations are conducted using realistic forcing data and are compared with the 1989 data of the north sea project sensitivity experiments are performed to test the physical and numerical impact of the heat flux parameterizations turbulence scheme and advective transport parameterizations of the surface fluxes with the monin obukhov similarity theory provide a relaxation mechanism and can partially explain the previously obtained overestimate of the depth mean temperatures in summer temperature stratification and thermocline depth are reasonably predicted using a variant of the mellor yamada turbulence closure with limiting conditions for turbulence variables the results question the common view to adopt a tuned background scheme for internal wave mixing two mechanisms are discussed that describe the feedback of the turbulence scheme on the surface forcing and the baroclinic circulation generated at the tidal mixing fronts first an increased vertical mixing increases the depth mean temperature in summer through the surface heat flux with a restoring mechanism acting during autumn second the magnitude and horizontal shear of the density flow are reduced in response to a higher mixing rate thermal and salinity fronts generate a seasonal circulation pattern in the north sea their impact on the horizontal temperature distributions is found to be in good agreement with the observations it is shown that in the absence of strong wind forcing both the vertical temperature distribution and the thermal circulation experience semimonthly variations in response to the spring neap cycle in tidal mixing at spring tides the surface mixed layers are shallower in agreement with observations at two mooring stations and the baroclinic circulation intensifies whereas the opposite occurs at neaps

wind driven motion near inner shelf fronts the response of inner shelf fronts to wind forcing is numerically studied here by using a three dimensional primitive equation model the front and its associated circulation are maintained by discharging light water near the surface and withdrawing heavier water from below along one vertical wall of a cyclic channel subsequently a spatially uniform alongshore wind is applied responses to an upwelling favorable wind and to a downwelling favorable wind are highly asymmetric upwelling favorable wind forces the light water pool to surface and expand offshore the pool is quickly detached from the source by the newly upwelled water even for a moderate plume and strong wind it is unlikely that a downwind coastal jet can develop and force the plume to turn against its natural direction of propagation recent observations of chesapeake plume support this finding downwelling favorable wind slowly narrows the plume inside which a strong coastal jet is developed for a density anomaly of finite alongshore extent the jet intensifies downstream reaches a maximum near the lead isopycnal and decays thereafter to compensate for the downstream acceleration strong onshore flow develops near the upstream edge of the plume if the wind persists the enhanced onshore flow further narrows the plume and triggers a second intrusion unstable waves develop behind the second intrusion they are however limited to small amplitudes by the prevailing onshore ekman drift

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

discrimination of the fault plane by waveform modeling a case study for moderate sized earthquakes in taiwan using nearby strong motion recordings and a finite source model we determine the focal mechanisms and identify the activated fault planes of two earthquakes of seismotectonic interest in taiwan the 15 december 1993 tapu m w 5 5 and the 17 july 1998 rueyli m w 5 7 these events occurred about 35 km apart in the same geological province which is characterized mainly by north northeast trending thrust faults as well as some strike slip faults they are moderate sized events that did not produce surface ruptures therefore the identification of fault planes appears a significant issue for this region accommodated in a complex tectonic environment and constitutes the main objective of this article both events were located in the vicinity of the chukou tachienshan fault system which is known to be dominated by east dipping thrust faults and both earthquakes resulted in almost pure reverse source mechanisms however we find that the two ruptures dip in opposite directions the northwest dipping fault of the tapu earthquake being in disagreement with the nearby fault system for the rueyli event we show that fault plane discrimination becomes effective when more than three stations are used and that certain stations contribute more than others we infer that this latter event occurred on one of the secondary east dipping thrust faults parallel to the tachienshan fault

regional crustal thickness variations of the peninsular ranges southern california we used the teleseismic receiver function technique to obtain a profile of the crustal thickness of the northern peninsular ranges california depth to the moho varies from 37 km beneath the western peninsular ranges batholith to 27 km at the western edge of the salton trough an average apparent dip of 10 to the west over a lateral distance of 60 km we previously obtained a similar result for a profile 100 km to the south a moho dip of 20 over 30 km lateral distance in both cases the moho depth variations do not correlate with topography of the eastern batholith but rather appear to parallel the trend of a boundary that separates compositionally distinct eastern and western terranes these observations suggest that a steeply dipping moho is a regional feature beneath the eastern peninsular ranges and that compensation is through lateral variations in crustal or upper mantle density rather than through an airy root

a possible causal relationship between the 1998 ruey li sequence and the 1999 chi chi earthquake in taiwan we have investigated the causal relationship between two recent taiwan events the 1998 m 6 2 ruey li and the 1999 m 7 6 chi chi earthquakes it is found that both earthquakes are generated by ruptures essentially along nearby sections of a large through going fault zone along the western foothills of taiwan the rapid termination in 10 days of the m 6 2 ruey li sequence is interpreted by the presence of an asperity in terms of the peikang basement high which dominates the tectonic deformation in central western taiwan we have carried out the relocation of the aftershocks their fault plane solutions plus stress inversion waveform source slip inversion coulomb stress change calculations and regional strain energy calculations for the ruey li sequence so as to examine its causal relationship from different angles we have concluded that as both events are results of the same regional stress accumulation due to the plate convergence the ruey li earthquake sequence would likely have escalated into a much larger rupture including the chelungpu fault had it not been for the presence of the peikang basement high

shear wave velocity model of the chukuo fault zone southwest taiwan from cross correlation of seismic ambient noise abstract the chia nan chiayi tainan area is in the southwestern taiwan and is located at the active deformation front of the collision of the eurasian continental plate and the philippine sea plate which causes complex folds as well as thrust fault systems in the area the chukuo fault zone is a boundary between the western foothill and the western coastal plain in the chia nan area the nature of the crustal structure beneath the fault zone especially the eastern part of the fault zone with mountain topography has not been well known in detailed due to lack of drilling data as well as its limitation in using other geophysical methods such as active source survey in this study we deployed an array with 11 broadband seismic stations to monitor the seismicity of the chukuo fault zone the array has recorded more than 1000 microearthquakes around this area it provides an opportunity to use p and s wave travel time data to investigate the both the crustal p and s velocity in the fault zone however due to the nature of the earthquake distribution the ray density is relatively low at depth between 0 and 7 km in addition the uncertainty of s wave reading for small earthquake also a limit in building precise s velocity profile thus we take the advantages of using cross correlation of seismic ambient noise to investigate crustal s velocity profile in the chukuo fault area especially in the mountain area where crustal faulting is a dominated phenomenon the results indicate that s wave velocity in the uppermost crust in the chukuo fault zone is shown to be slower than previous studies a low velocity layer exists at depth between 1 and 2 km in the east of the chukuo fault the low s velocity is related to a highly fractured upper crust due to intensive deformation caused by the orogenic process

modified frequency wavenumber algorithm for regional seismograms using filon s quadrature modelling of lg waves in eastern north america summary the main focus of this study is to develop an efficient frequency wavenumber code to synthesize high frequency say 10 hz regional seismograms up to a distance of more than 1000 km in a medium consisting of a large number of crustal layers and investigate effects of regional waveguides on lg waves in this newly developed code the frequency wavenumber response is evaluated by using the filon s quadrature at high frequency and the polynomial approximation of bessei s functions implementing trapezoidal integration at low frequency this has reduced the computation time by several folds compared to the time needed for the trapezoidal integration using this algorithm both broad band seismograms at harvard and short period ectn eastern canadian telemetered network seismograms from 1988 november 25 saguenay earthquake were successfully modelled a moment magnitude of 5 9 was used initially an average waveguide was calibrated to successfully excite the regional pnl and snl waves at hrv its near surface structure constrained by matching the recorded frequency content the crust mantle transition zone was constrained by modelling the strength of the broad band snl waves consisting of sn ssmsnsmssms and ssmssms phases relative to the initial pnl waves the model was modified to consist of thin layers with an alternating high and low velocity distribution to match the high frequency ectn lg waves observed from 300 to 600 km away in addition the effect of a near surface irregular receiver structure on the duration of lg waves was also investigated by convolving its response with the regional lg seismograms initial study suggests this method as viable for modelling the entire lg waves including the coda numerical studies suggest that the shapes and peak amplitudes on lg seismograms depend more strongly on the source depth anelasticity and crustal waveguide than do the pnl seismograms

complex rayleigh waves resulting from deep sedimentary basins abstract two earthquakes in northern mexico recorded by trinet stations in southern california show complex surface wave propagation through the salton trough stations contained in a narrow corridor extending from the salton trough north towards the central valley california show recorded waveforms with two rayleigh wave arrivals the time separation between the rayleigh wave arrivals is nearly constant beyond the northern edge of the salton trough we are able to reproduce both arrivals with a simple 1 d model of a low velocity deep basin in the salton trough the slower rayleigh wave is propagating within the sediments of the basin while the faster rayleigh wave propagates mainly in faster basement rock further we reproduce the constant time separation through the implementation of a simple 2 d model this 2 d model has a low velocity deep basin near the source and a crust without a basin at distances greater than 200 km

spring summer frontogenesis at the mouth of block island sound 2 combining acoustic doppler current profiler records with a general circulation model to investigate the impact of subtidal forcing 1 the use of a one layer inverse model to improve the fidelity of a three dimensional primitive equation model is explored in the region just south of block island sound in the middle atlantic bight the inverse model based on the theory of bennett and mcintosh produces a solution to the linear shallow water equations that minimizes model data misfit and the resulting open boundary velocity variance the massachusetts institute of technology general circulation model is configured regionally and includes realistic tidal forcing surface warming and wind stress as well as climatological buoyancy gradients between the estuary and the shelf a 15 day record from two acoustic doppler current profiler moorings deployed in spring 2000 south of the entrance of block island sound allows assessment of the forward model and provides data to improve subtidal boundary conditions model data misfit decreases notably for both the depth averaged and vertically sheared flow indicating that improvements to the depth averaged circulation positively impact the three dimensional circulation in this shallow shelf environment the role of wind stress in modulating the position of the buoyant plume that characterizes the regional hydrography is also examined a distinctly three dimensional circulation is observed with a growing offshore plume at the mouth of the sound during northward stress followed by rapid alongshelf advection during periods of southwestward or weak winds although on the timescales considered here wind stress forcing dominates the buoyancy driven motion it is overwhelmed by the tidally driven rectified flow just south of montauk point which provides the inshore boundary of the front even in downwelling favorable conditions

mapping of the high frequency source radiation for the loma prieta earthquake california waveform inversion has been successfully applied to map the spatial distribution of earthquake fault slip from strong motion records but its application has been restricted to a relatively lower frequency range of the data 5 hz generation is incoherent between neighboring patches on the fault we formulated an analytical model for calculating the envelope time history of the squared high frequency displacement seismogram this envelope time history is found through the line integration of the squared energy radiation intensity over the isochron using a recursive stochastic inversion scheme we determined both the lower frequency slip distributions from the low pass filtered strong motion displacement data and the higher frequency energy radiation intensity from the envelope of the squared high frequency displacement data on an extended fault for the 1989 loma prieta earthquake respectively our resulting lower frequency slip model shows that there are two main slip patches and little slip directly updip of the hypocenter one major slip zone located 10 km northwest of the hypocenter with predominant dip slip and the other located 12 km southeast of the hypocenter with predominant oblique slip our preliminary results of the high frequency energy radiation intensity inversion indicate three large high frequency sources and few smaller ones located on the outer periphery of the large slip zones this suggests that the high frequency energy sources are located along or near the boundaries of large slip zones which is consistent with the theoretical consideration that high frequencies are primarily generated from the rupture stopping areas or places with large slip variation

crustal thickness of the peninsular ranges and gulf extensional province in the californias we estimate crustal thickness along an east west transect of the baja california peninsula and gulf of california mexico and investigate its relationship to surface elevation and crustal extension we derive moho depth estimates from p to s converted phases identified on teleseismic recordings at 11 temporary broadband seismic stations deployed at 318n latitude depth to the moho is 33 63 km near the pacific coast of baja california and increases gradually toward the east reaching a maximum depth of 40 64 km beneath the western part of the peninsular ranges batholith the crust then thins rapidly under the topographically high eastern peninsular ranges and across the main gulf escarpment crustal thickness is 15 18 62 km within and on the margins of the gulf of california the moho shallowing beneath the eastern peninsular ranges represents an average apparent westward dip of 258 this range of moho depths within the peninsula ranges as well as the sharp east west gradient in depth in the eastern part of the range is in agreement with earlier observations from north of the international border the moho depth variations do not correlate with topography of the eastern batholith these findings suggest that a steeply dipping moho is a regional feature beneath the eastern peninsular ranges and that a local airy crustal root does not support the highest elevations we suggest that moho shallowing under the eastern peninsular ranges reflects extensional deformation of the lower crust in response to adjacent rifting of the gulf extensional province that commenced in the late cenozoic support of the eastern peninsular ranges topography may be achieved through a combination of flexural support and lateral density variations in the crust and or upper mantle

ocean currents along a nearshore frontal zone on the continental shelf of the southeastern united states abstract river and estuarine discharges mix to form a frontal zone along a 400 km length of the coast of the south eastern united states the frontal width extends from the coast outward 10 20 km two ships anchored across the frontal zone during autumn obtained simultaneous hourly profiles of currents and density for five consecutive tidal cycles the frontal zone contained a baroclinic coastal current flowing southward the flow was strongly convergent more water entered on the coastal side than exited on the seaward side much of the inflow was apparently turned southward thus the frontal zone acted as a dynamic barrier that inhibited the advection of mixed river discharge farther offshore a thermal wind relationship suitably predicted vertical shear through the frontal zone bottom friction seemed to play only a secondary role data suggest that the baroclinic coastal current was modified by the presence of a barotropic current seaward of the frontal zone the force responsible for the barotrop

crustal thickness variations beneath the peninsular ranges southern california we investigate the crustal thickness under the peninsular ranges using p to s converted phases of teleseismic body waves recorded on a temporary broadband seismometer array and isolated by the receiver function method ps minus p times at sites west of a compositional boundary that separates the peninsular ranges batholith into east and west zones indicate a relatively flat deep moho ps minus p times at sites east of the compositional boundary decrease eastward moho depth estimates made from the ps delays and crustal velocities from seismic tomography indicate a relatively constant 36 to 41 km thick crust in the western zone in the eastern zone the crust thins rapidly from 35 km thick at the compositional boundary to 25 km at the edge of the salton trough a lateral distance of 30 km the lack of correlation between topography and moho depths suggests compensation via lateral density variations in the lower crust or upper mantle we propose that the compositional boundary decouples the eastern and western portions of the batholith and that the eastern portion has thinned in response to regional miocene extension or salton trough rifting or both

preface to the 1999 chi chi taiwan earthquake dedicated issue the 1999 chi chi taiwan earthquake m w 7 6 struck central western taiwan at 1 47 a m local time on 21 september or at utc 17 47 20 september the causal fault was immediately identified to be the mapped active chelungpu thrust fault as it produced a remarkable 100 km long surface break with fault scarps as high as 8 m in places the nucleation point of this long rupture was defined by an epicenter at 120 82 e and 23 85 n with a focal depth of 8 km a first motion solution was reported to be striking n20 e dipping 30 se with an average rake of 85 chang et al 2000 the event was officially named after the nearest town chi chi by the central weather bureau cwb shin 2000 this was the most devastating earthquake to hit taiwan in modern times the chi chi earthquake and its energetic aftershock sequence inflicted a casualty toll of 2435 and an estimated us 4 billion property loss tsai et al this issue this catastrophe was nonetheless fortunate in several aspects the mainshock struck when schools were not in session the loss of student lives would have been unthinkable in view of the total collapse of hundreds of school buildings in emergency response it was fortunate that the cwb completed in 1995 the taiwan rapid earthquake information release system that was capable of electronically releasing pertinent information within a couple minutes of an earthquake occurrence rapid earthquake information greatly facilitated the disaster relief missions and it mitigated further secondary losses scientifically because of the completion of the taiwan strong motion instrumentation program tsmip the occurrence of the chi chi earthquake sequence has resulted in the richest seismological data recovery in the world to date in both quality and quantity these data were open to the world with a data cd of the

on the transport of buoyant coastal plumes abstract the role of discharge conditions and shelf geometry on the transport of coastal plumes is studied with a fully nonlinear primitive equation hydrodynamic model the physical setting is an estuarine channel with a small discharge rossby number by simulating different discharge magnitudes buoyant plumes are shown to be succinctly described by a simple coastal front model three results emerge from the model analysis first the plume transport is given by t 0 g ph2 2f where 0 is a parameter dependent on the ratio of the front and the plume widths g p is the plume reduced gravity h is the plume maximum depth and f is the coriolis parameter second this model links the plume transport directly to upstream river conditions with t qr where qr is the river outflow and is a parameter that relates to entrainment the geometry of the plume front and shelf slope and the fraction of freshwater carried downshelf third these equations reduce to analytic results previously established for

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

biologically induced circulation at fronts consider a frontal region that has high phytoplankton biomass on one side and low biomass on the other irradiance penetrates deeply through the water column on the low biomass side but is attenuated nearer the surface on the biomass rich side because of absorption by phytoplankton thus the near surface water is heated more on the biomass rich side than on the clearer side resulting in lower density surface water on the biomass rich side at greater depths the situation is reversed with lower density water occurring on the biomass poor side we model this situation and examine the resulting perturbations to the frontal circulation our aim is to provide an order of magnitude estimate of the feedbacks from the biological component of the ecosystem to the current field the model consists of the steady state momentum equations including coriolis pressure gradient and viscous effects we compute induced vertical velocities of up to 0 2 mm s 1 commensurate with field measurements and previous modeling estimates of vertical velocities at fronts the horizontal along frontal velocities are of order 2 cm s 1 or less and so will not represent a major contribution to the overall flow field however such values are certainly not insignificant

the gulf stream and its frontal structure a quantitative representation abstract the structure of the gulf stream current is associated with the quasi permanent density front in the western north atlantic the lighter mass of warmer but saltier water of the sargasso sea is separated from the slope water by inclined isopycnals that form the front recent satellite altimeter measurements have also revealed a well defined sea surface height change across the front in this paper a model of the gulf stream cross sectional density and current structure is presented using the complete dynamical and mass conservation equations the model postulates a forcing at the interior ocean boundary by a cross stream ageostrophic circulation with inflow of light water in the upper ocean and a return flow at greater depths the model gulf stream is found to develop after initial geostrophic adjustment of several inertial periods in the quasi steady state the normalized structural results constitute a single representation of the structure of all gulf stream sections i e all sections ar

frontal instabilities of baroclinic oceanic currents with applications to the gulf stream abstract the frontal instability of major baroclinic ocean currents such as the gulf stream is numerically studied here using a three dimensional primitive equation model the model current is driven by a buoyant discharge near the surface from the light water side of the front subsequent geostrophic adjustment produces a baroclinic analysis in which a prescribed mean current is subject to eddy dissipation with no provision for its maintenance at low latitudes small amplitude unstable waves are generated the eddy fluxes are shown to be largely upgradient and responsible for the maintenance of the front at higher latitudes the model produces anticyclonic barotropic instability the triggering mechanism for instability is related to the outward surge of the front during the initial stage of geostrophic adjustment which in turn is related to the inertial oscillations of surface isopycnals the outward surge is surface trapped to trigger instability the surge must be shallow and intense in lower lati

ruptures in a highly fractured upper crust the 1999 chiayi m 6 4 earthquake sequence that occurred about one month after the 1999 chi chi m 7 6 earthquake is remarkably similar to the ruey li m 6 2 earthquake sequence which occurred one year before the big chi chi event both sequences were energetic and short in duration with events confined in small source volumes and the fault plane orientation of individual events in these sequences varied abruptly during the development of the dynamic rupture both occurred on the southern extension of the large chelungpu rupture causal to the chi chi event and in an area of highly fractured upper crust we consider that this faulting behavior is due to the rupture induced dynamic stress that interacts with the pre existing highly fractured upper crust and continually perturbs the local stress field this phenomenon appears to be typical of particularly active plate boundary regions with a highly fractured upper crust under intense tectonic loading

stress tensor inversion for the chi chi earthquake sequence and its implications on regional collision we study the stress regime associated with the 1999 chi chi taiwan earthquake sequence broadband waveforms of 115 events recorded by the broadband array in taiwan for seismology bats are analyzed to determine their focal mechanisms depths and seismic moments these solutions are then used to invert for the reduced stress tensor using the algorithm of angelier 1998 we perform the stress inversion in three different ways first we aim at defining in the first approximation a uniform stress field by taking the entire dataset as a whole the inversion result shows a stress regime with the maximum compression along the direction of n119 e remarkably consistent with the direction of the relative motion between the philippine sea plate and eurasia this solution is stable and varies little in terms of the orientations of stress axes within 8 and the ratio of stress differences between 0 26 and 0 44 in the second approximation we aim at explaining more subtle features of our observations by considering differently built subsets we divide the dataset according to different seismogenic structures as identified by kao and chen 2000 our results yield rather similar results axes of maximum compressive stress fall in the range of n99 138 e in terms of trends and in the range of 1 17 in plunges for all subsets except a small group in the footwall furthermore we observe a systematic clockwise change in the direction of compression from south to north representing the more general fan shaped pattern of stress trajectories that characterizes the whole collision zone finally the inversion is performed for the three main types of faulting that is reverse strike slip and normal types the axis of maximum compression for the reverse mechanisms has the same direction as that for the strike slip whereas the normal type subset yields a n54 e extension that trends obliquely to the compression the fact that the stress regime associated with the chi chi aftershock sequence is similar to that derived from geological and seismic data in the region implies that the stress regime corresponding to the regional collision in taiwan is responsible for the occurrence of the chi chi earthquake moreover systematic secondary variation in the state of stress is observed for different types of focal mechanisms and seismogenic structures such variation may play an important role in producing local stress concentration that leads to the nucleation of disastrous earthquakes in central taiwan manuscript received 12 january 2001

static stress transferred by the 1999 chi chi taiwan earthquake effects on the stability of the surrounding fault systems and aftershock triggering with a 3d fault slip model we investigated the effect of changes in static stress caused by the 1999 m l 7 3 chi chi earthquake on the surrounding fault systems in taiwan we examined this problem from two perspectives first we applied a statistical test for the hypothesis of static stress triggering by comparing the distributions of the coulomb failure stress changes on the focal mechanism nodal planes both before and after the mainshock since preshocks could obviously not have been affected by the mainshock the deviation of these distributions determined the significance of static stress triggering second we calculated the coulomb failure stress changes on the chukou and meishan faults in specified slip directions to evaluate the stability of these two active faults after the chi chi earthquake the static stress changes were obtained by applying the theory of dislocation in an elastic half space to a 3d rupture model derived from the surface displacements associated with the chi chi earthquake our results showed that the distributions of the coulomb failure stress changes for events preceding and following the chi chi earthquake were significantly different yet consistent with the hypothesis of static stress triggering we also found that both the chukou and the meishan faults were in stress shadows at depths shallower than 10 km however because a number of aftershocks fell in these stress shadows it is possible that other mechanisms such as dynamic stress were responsible for triggering these events manuscript received 6 november 2000

the rhone river plume in unsteady conditions numerical and experimental results surface current maps obtained in the plume of the rhome river by two vhf radars gave rise to comparisons with simulations produced by a primitive equation model two situations were investigated one in steady conditions the other with a variable wind the model gives a good estimation of current direction and velocity not only in the plume itself but also in the surrounding water the model reveals the mechanisms involved in the plume dilution and more generally in the renewal of surface water near the coast when the wind changes direction the sensitivity of the model results to the upper boundary condition for turbulent kinetic energy is also shown

observations of the response of a buoyant estuarine plume to upwelling favorable winds 1 the response of the buoyant plume from the chesapeake bay to upwelling favorable winds and during conditions of low freshwater discharge is examined using an offshore array of bottom deployed acoustic doppler current profilers adcps as well as ancillary data during periods of northward upwelling favorable winds the observed response is strong offshore flow u 0 2 m s in a shallow layer of plume water and a rapid reversal of the near surface flow from the usual southward flow associated with inertial turning of the plume the offshore migration of the plume front is deduced from sequential arrivals at the various mooring locations the orientation of the plume front is approximately alongshore north south suggesting that the upwelling response is two dimensional a plume thickness calculated from the adcp data is relatively constant at about 4 m as the plume moves across the moorings the offshore propagation speed of the plume front of about 0 20 m s matched the advection speed predicted by ekman theory using the calculated plume layer thickness the mooring velocity profiles provide a view of the across plume structure in a reference frame fixed to the front this reveals a pattern of circulatory motion with sinking of plume and ambient water at the front and an ascending motion occurring inshore suggesting a recirculation between the seaward and inshore portions of the plume these results are consistent with recent model simulations of the response of a river plume during upwelling favorable winds

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

discrimination of fault planes from auxiliary planes based on simultaneous determination of stress tensor and a large number of fault plane solutions it is well known that there is a large difference in focal mechanism solutions of events even if they occur within a certain small area considering earthquake generating stress to be uniform in a studied area we developed a new method for simultaneously determining the stress tensor and the orientation of fault planes for many events by the use of polarity data of p waves first the number of inconsistent stations is calculated for various values of three parameters in the focal mechanism solution with intervals of 10 this calculation is made for all the events used then four parameters defining the stress tensor are determined by a grid search using the data calculated above with the assumption that the slip direction in the fault plane should be parallel to the direction where the shear stress on the plane becomes maximum a numerical test is made by applying the present method to an artificial data set composed of 25 events with 25 25 readings it is found that a stable solution of the stress tensor is determined by the inversion and about 60 of fault planes are distinguished from auxiliary planes there are no events whose auxiliary planes are incorrectly determined as fault planes this method is used to estimate the earthquake generating stress in the aftershock area of the 1984 western nagano prefecture earthquake central japan the results obtained show that the maximum principal stress is horizontal in the direction of n80 w s80 e there are strike slip events having fault planes almost perpendicular to the fault plane of the main shock

the dynamics of oceanic fronts part ii shelf water structure due to freshwater discharge abstract in part i of this series a framework was introduced for the study of oceanic frontal dynamics the dynamics was studied by posing an initial value problem starting with a near surface discharge of buoyant water with a prescribed density deficit into an ambient stationary fluid of uniform density an essential aspect of the framework was the identification of the proper length scales an inertial length scale l0 a buoyancy length scale h0 and a diffusive length scale hv in part i the horizontal and vertical dimensions were scaled by l0 and h0 respectively and two dimensionless parameters were formed viz ro l0 h0 and e hv h0 2 it was shown in part i that under this scaling the normalized equations depended on e only for ro sufficiently large the solution for e small i e for the almost inviscid case was given in part i and the equilibrium state was discussed in a frame of reference in which the front was stationary in this paper we present the solution for large e it will b

stress tensor and fault plane solutions for a population of earthquakes an algorithm for the simultaneous estimation of the orientation and shape of the stress tensor and the individual fault plane solutions for a population of earthquakes is studied it corresponds to a synthesis of the methods used by brillinger et al 1980 to obtain focal mechanisms and by armijo and cisternas 1978 for stress tensor analysis in microtectonics the input data are the polarities of the p arrival and take off angles for the set of source station pairs the method distinguishes in general which one of the nodal planes corresponds to the fault and gives the direction of the slip the application to the aftershock sequence of the 1980 arudy earthquake western pyrenees shows that the observations may be explained by a single stress tensor producing a n32 e extension with a likelihood of 95 per cent

seismic waveform modeling in the los angeles basin the proximity of several recent earthquakes to the los angeles sedimentary basin provides an opportunity to isolate the effects of the basin on wave propagation the 4 october 1987 aftershock m l 5 3 of the whittier narrows sequence and the 28 june 1991 sierra madre mainshock m l 5 8 are on a similar azimuth to stations overlying the deepest part of the basin a distinctive feature of records from basin stations recording the 4 october after shock is the large amplitude of multiple s ss etc the multiples have up to twice the amplitude of the direct s phase on the tangential component at such a short range less than 25 km a horizontal seismic velocity gradient is needed to turn rays rapidly enough for large amplitude multiples to form a forward modeling approach is employed using finite difference numerical techniques that produce double couple point source solutions a model based on a recent geologic cross section constructed for the east edge of the los angeles basin generates more phases than are seen in the seismic records simpler models based on dipping layers with low shear velocities in the top few layers fit the data better the seismic velocity depth and dip of the layers are varied to fit the timing between the direct p the direct s and the first s multiple the timing and amplitude of the direct and first multiple s pulses are well modeled though the phase of the first multiple does not match the data including a steeply dipping west edge in the basin model has little effect on the synthetic waveforms except at distances near that basin edge the amplitude of ss is greatest in the deepest part of the basin where it is two times larger than direct s the coda duration increases from 8 sec to more than 20 sec from the ne to the sw the sierra madre mainshock occurred about 25 km to the ne of the whittier narrows sequence the model for whittier narrows was extended to this distance with a shallow basin between whittier and the sierra madre hypocenter to simulate the san gabriel sedimentary basin phases generated by the edge of the deep basin continue to dominate the synthetic waveforms but this model generates a lengthy coda this study shows that specific phases with frequencies up to 1 hz that have traveled through deep sedimentary basins can be explained by two dimensional seismic velocity models

idealized three dimensional modeling of seasonal variation in the alaska coastal current 1 seasonal variation of the buoyancy and downwelling wind forced alaska coastal current acc and the fate of freshwater contained in it is considered using idealized analytical and numerical models of the acc formed from a half line source of buoyant inflow the coastal current initially develops two dimensionally but becomes three dimensional from a balance between coastal influx of buoyancy and its downstream transport which leads to a coastal current depth limit hmax 1 2 where x is along shelf distance q is the line source strength for unit length f is the coriolis frequency and g is the reduced gravity of the buoyant inflow this limit is unchanged under downwelling wind stress and is reached on timescales of less than 1 month for the acc the coastal current width is roughly constant in x and increases in time at the same rate as the two dimensional solution imposition of a downwelling wind stress results in an approximate balance among wind stress and along and cross shelf momentum advection so that the current width is reduced to ywind ld 1 2 where ld is the rossby radius of deformation is the wind stress and 0 is a reference density waves eddying motions eventually grow in the half line source coastal current with wavelengths proportional to the coastal current width and with a downstream phase speed slower than the maximum current speed these features cause an offshore flux of buoyant water a broader coastal current and further accumulation of buoyancy on the shelf increasing downwelling wind stress reduces the effects of the instabilities continual accumulation of buoyancy on the shelf occurs during all model runs but is nearly absent under maximum winter downwelling wind stress it is suggested that freshwater accumulation on the shelf during spring summer and fall may be largely lost downstream during winter

source process and tectonic implication of the january 20 2007 odaesan earthquake south korea abstract the source process for the 20th of january 2007 mw 4 5 odaesan earthquake in south korea is investigated in the low and high frequency bands using velocity and acceleration waveform data recorded by the korea meteorological administration seismographic network at distances less than 70 km from the epicenter synthetic green functions are adopted for the low frequency band of 0 1 0 3 hz by using the wave number integration technique and the one dimensional velocity model beneath the epicentral area an iterative technique was performed by a grid search across the strike dip rake and focal depth of rupture nucleation parameters to find the best fit double couple mechanism to resolve the nodal plane ambiguity the spatiotemporal slip distribution on the fault surface was recovered using a non negative least square algorithm for each set of the grid searched parameters the focal depth of 10 km was determined through the grid search for depths in the range of 6 14 km the best fit double couple mechanism obtained from the finite source model indicates a vertical strike slip faulting mechanism the nw faulting plane gives comparatively smaller root mean squares rms error than its auxiliary plane slip pattern event provides simple source process due to the effect of low frequency that acted as a point source model three empirical green functions are adopted to investigate the source process in the high frequency band a set of slip models was recovered on both nodal planes of the focal mechanism with various rupture velocities in the range of 2 0 4 0 km s although there is a small difference between the rms errors produced by the two orthogonal nodal planes the sw dipping plane gives a smaller rms error than its auxiliary plane the slip distribution is relatively assessable by the oblique pattern recovered around the hypocenter in the high frequency analysis indicating a complex rupture scenario for such moderate sized earthquake similar to those reported for large earthquakes

the enigma of the arthur s pass new zealand earthquake 2 the aftershock distribution and its relation to regional and induced stress fields the aftershock distribution of the 1994 arthur s pass earthquake mw6 7 is unusual for a reverse faulting event in that it extends 12 km nnw and 30 km sse of the actual fault plane which strikes ne sw we have used several methods to infer the regional stress field in the region including geodetic results earthquake mechanisms and inversion of p wave polarity data for the stress tensor orientation the inversion method is new and does not require the focal mechanisms of the events used it also incorporates the coulomb failure criterion all results point to a stress field favoring strike slip faulting not thrusting with near horizontal 1 and 3 principal axes striking at 298 and 28 using dislocation theory we calculate the stress induced by the arthur s pass earthquake and its largest aftershock a strike slip event and add this to the regional field there is a fair correspondence between the hypocenters of aftershocks away from the mainshock fault plane and regions of high induced coulomb failure stress cfs on optimally oriented fault planes however there are regions of high induced cfs that are devoid of aftershocks it appears that earthquake slip in this region of oblique 19 plate convergence is as observed elsewhere partitioned into components parallel and perpendicular to the plate margin most of the slip is parallel as occurs on the nearby dextral alpine fault the boundary between the pacific and australian plates however occasional reverse events such as the arthur s pass earthquake account for at least some of the perpendicular component of slip and the uplift that produced the southern alps

numerical simulations of a buoyancy driven coastal countercurrent off vancouver island abstract a three dimensional prognostic numerical model has been developed to study the ocean circulation around vancouver island british columbia in a series of simulations the model is applied to examine the role of buoyancy forcing in the dynamics of the summer coastal countercurrent found off the west coast of vancouver island the forcing is provided by the fraser river discharge into the strait of georgia an estuarine circulation establishes itself in juan de fuca strait from which a distinctive right bounded current is formed and advances along the coast sensitivity studies are conducted to determine the robustness of this current to initial conditions opposing wind enhanced vertical mixing and grid resolution finally various characteristics of the numerically modeled coastal flow are compared with observations

ray tracing for earthquake location in laterally heterogeneous media we propose a technique for the fast solution of ray tracing in three dimensional laterally heterogeneous media analytical solutions for both ray tracing and paraxial ray tracing for a medium with an arbitrary gradient of the square of slowness are used to develop a systematic procedure to construct exact solutions complex heterogeneous media are divided into tetrahedral finite elements inside which the square of the slowness has a simple linear distribution arbitrarily complex media may be studied by judicious choice of the elements we develop appropriate boundary conditions for interfaces with zeroth or first order velocity discontinuities two point ray tracing is performed by a newton method based on paraxial ray theory with a few iterations the ray trajectories through the source and the observer are calculated with the same overall speed as with ray bending the paraxial method has the additional advantage that it can be used to identify caustics and to separate travel time branches we used the previously described method to replace the ray tracing algorithm of the hypo71 program probably the most commonly used earthquake location program the ray tracing routine may be easily modified to adapt it to other earthquake location environments with the modified hypo71 program we relocated the aftershock sequence of the november 23 1980 irpinia italy earthquake the introduction of a low velocity zone proposed by different authors shifts aftershock locations to the ne this horizontal displacement of hypocenters is not very sensitive to the thickness of the low velocity zone

circulation mixing and exchange processes in the vicinity of tidal inlets a numerical study the circulation in the vicinity of an idealized tidal inlet connecting a continental shelf and a coastal sound is examined the circulation is forced by an m2 tide and a weakly buoyant discharge the buoyant discharge forms a plume in the coastal ocean and induces a distinct anticyclonic circulation at the plume edge that is maintained throughout the tidal cycle we focus on the plume s onset and its evolution over 5 10 tidal cycles over the timescales considered the plume was roughly circular slightly skewed in the along coast direction the model solution yielded high vertical ekman numbers e o 5 in the vicinity of the inlet mouth decreasing seaward from the inlet to an order of magnitude smaller e 0 25 at the seaward edge of the plume passive particles released in the region seaward of the inlet mouth were used to describe the exchange between the coastal region and the inlet a marked asymmetry between ebb and flood flows is observed in the vicinity of the inlet with jet like ebbing currents and weaker potential flow like flooding currents over a tidal cycle net exchanges between the coastal ocean and the inlet are found to be spatially and temporally dependent that is particle trajectories depend on the release point and the time of the release in the tidal cycle the near inlet residual circulation shows significant differences in the absence of stratification