centrifuge modeling of dynamic response of high fill slope by using generalized scaling law abstract the development of physical modeling for mega engineering prototypes requires sophisticated use of the capability of current centrifuge facilities in this study centrifuge model tests were performed to investigate the dynamic response of high fill slope by using generalized scaling law in an improved way firstly the two stage generalized scaling law was partly re organized by considering the separate similitude of dynamic strain for the virtual 1 g field to solve the inherent problem of the scaling of soil strain when the derived prototype strain exceeded some important thresholds then a series of centrifuge model tests were conducted under different centrifugal accelerations to model a high fill slope with a prototype height of 100 m by the approach of modeling of models dynamic responses under sine waves with different amplitudes were monitored and the re organized generalized scaling law was validated for nonlinear dynamic response with shear strain less than 1 it was observed that site amplification of high fill slope was affected by both soil nonlinearity and soil strata non uniformity where soil nonlinearity led to smaller acceleration amplification under stronger excitation and soil strata non uniformity contributed to the acceleration de amplification and larger displacement amplification at the weak sublayer the large dynamic displacement near the slope surface consequently contributes to the development of lateral cracks after strong shaking seismic response under earthquake motions revealed that high fill slope with inclined non uniform strata was characterized by multiple vibration modes and lower fundamental frequency which was more vulnerable to earthquake motions with abundant frequency components this study provides a more physically meaningful use of the similitude of dynamic stress strain relation in centrifuge modeling for large scale geotechnical problems under dynamic loadings

simplified method for evaluating seismic stability of steep slopes steep slopes composed of weakly cemented granular soils are common along the pacific coast of north america these slopes standing at angles of 30 to near vertical are observed to be prone to seismically induced failures the steep topography of these slopes makes them particularly susceptible to amplification of seismic waves and the brittle nature of these materials make deformation based stability analyses inappropriate in this paper a simplified method is presented for assessing the seismic stability of these slopes utilizing average seismic coefficients to account for the effect of topography this procedure is based on a review of field and laboratory observations as well as analyses carried out using generalized consistent transmitting boundaries the results of the analyses indicate that the amplification due to the soil column in the free field behind the crest dominates the response the amplification due to topography on the other hand shows little variability and is on the order of 50 the proposed procedure presented herein gives practicing engineers a method to conduct stability analyses that better represents the actual field conditions for these steep brittle slopes

a parametric numerical study of the interaction between seismic waves and landslides for the evaluation of the susceptibility to seismically induced displacements this study presents the results of a numerical analysis investigating the influence of the interaction between seismic waves and slopes on seismically induced displacements of existing landslides three step like slope geometries with dip angles varying from 10 degrees to 45 degrees and two landslide mechanisms were considered 54 multi frequential dynamic equivalent signals were applied to the models and were derived according to the lema des approach based on the accelerometric records of the european strong motion and cosmos databases the simulations were performed using a finite difference approach that assumed different viscous rheologies and considered different values for both the dynamic properties and the mechanical parameters the interaction between seismic waves and landslides was analyzed using frequency ratios defined for characteristic periods associated with the landslide dimensions i e thickness and length and the seismic input a regression analysis was also performed to derive correlations between seismically induced displacements and the frequency ratios the values of the arias intensity and the critical accelerations the obtained results reveal that a the seismically induced displacements increase with the seismic amplification induced by the landslide mass b the coupling of a 2d interaction between seismic waves and slope with the 1d resonance of the landslide mass which is strictly related to the high impedance contrasts between the landslide mass and the substratum significantly affects the seismically induced displacements and c the seismically induced displacements increase more intensely with increasing slope angle for a rototranslational mechanism with respect to a translational one moreover the seismically induced displacements that can be expected from the here obtained results can significantly differ from those computed by the sliding block stability analyses based on newmark s method

developments in the characterization of complex rock slope deformation and failure using numerical modelling techniques recent advances in the characterization of complex rock slope deformation and failure using numerical techniques have demonstrated significant potential for furthering our understanding of both the mechanisms processes involved and the associated risk this paper illustrates how rock slope analyses may be undertaken using three levels of sophistication level i analyses include the conventional application of kinematic and limit equilibrium techniques with modifications to include probabilistic techniques coupling of groundwater simulations and simplistic treatment of intact fracture and plastic yield such analyses are primarily suited to simple translational failures involving release on smooth basal rear and lateral surfaces where the principle active damage mechanisms are progressive failure and or asperity breakdown level ii analyses involve the use of continuum and discontinuum numerical methods in addition to simple translation level ii techniques can be applied to complex translational rock slope deformations where step path failure necessitates degradation and failure of intact rock bridges along basal rear and lateral release surfaces active damage processes in this case comprise not only strength degradation along the release surface e g asperity breakdown but also a significant component of brittle intact rock fracture level iii analyses involve the use of hybrid continuum discontinuum codes with fracture simulation capabilities these codes are applicable to a wide spectrum of rock slope failure modes but are particularly well suited to complex translation rotational instabilities where failure requires internal yielding brittle fracturing and shearing in addition to strength degradation along release surfaces through a series of rock slope analyses the application of varied numerical methods are discussed particular emphasis is given to state of the art developments and potential use of level iii hybrid techniques d 2005 elsevier b v all rights reserved

monitoring numerical modelling and hazard mitigation of the moscardo landslide eastern italian alps abstract the moscardo torrent basin eastern italian alps is a high risk site since a large roto translational landslide might dam the torrent with the consequence of increasing the possibility of large debris flow events creating a threat for the infrastructures and the socio economic activities of the villages that dot the valley below the landslide whose volume is estimated 2 million m3 has been monitored since 2006 with inclinometers electric piezometers and a gps network the velocity along the entire body of the landslide averages 1 0 1 5 cm per month the shear surface develops at depths varying from 9 to 10 m to 55 62 m while the groundwater table is almost constant throughout the year despite a cumulative rainfall of the area that usually reaches 2000 mm year the movements were simulated in a numerical model in order to estimate the stabilization effect obtained by different types of possible countermeasures the simulation was carried out using flac 2d with creep modelling visco elasto plastic model of the medium in the sliding zone was assumed allowing to determine the relation between time and displacement a 10 year displacement trend starting from the initial situation of 2006 was simulated moreover seismic conditions were taken into consideration with a quasi static approach by applying a horizontal acceleration the numerical model was built and validated on the basis of the data retrieved from geological investigations as well as from inclinometric and gps measurements the results show that an accurate and well planned multidisciplinary approach can help the decision makers in the choice of the most effective engineering solution for the mitigation of landslide hazard and risk

ventilation of the subtropical north pacific the shallow salinity minimum abstract the shallow salinity minimum of the subtropical north pacific is shown to be a feature of the ventilated wind driven circulation subduction of low salinity surface water in the northeastern subtropical gyre beneath higher salinity water to the south causes the salinity minimum variation of salinity along surface isopycnals causes variations in density and salinity at the minimum a model of ventilated flow is used to demonstrate how the shallow salinity minimum can arise the model is modified to account for nonzonal realistic winds it is also extended to examine the three dimensional structure of the western shadow zone the boundary between the subtropical and subpolar gyres is given by the zero of the zonal integral of ekman pumping the western shadow zone fills the subtropical gyre at the base of the ventilated layers and decreases in extent with decreasing density for parameters appropriate to the north pacific the eastern shadow zone is of very limited extent observations of salini

long term acoustic monitoring at north sea well site 22 4b highlights first study using long term passive acoustic monitoring of methane seeps at well blowout site 22 4b seep acoustic temporal variations correlated with ocean tides major acoustic transient event recorded on 8 december 2011 with high temporal resolution abstract marine seeps produce underwater sounds as a result of bubble formation and fragmentation upon emission from the seabed the frequency content and sound levels of these emissions are related to bubble size distribution and emission flux providing important information on methane release from the seafloor long term passive acoustic monitoring was used to continuously record seep sounds over a 7 month period within the blowout crater at the abandoned well site 22 4b in the central north sea also recorded were water column fluid velocities and near seafloor water conductivity temperature and pressure acoustic signatures were primarily from 1 to 10 khz key features were relatively broad spectral peaks at about 1 0 1 5 2 2 3 1 3 6 and 5 1 khz temporal variations in spectral levels were apparently associated with tides the recordings also documented a series of major episodic events including a large and persistent increase 10 db in overall sound levels and spectral broadening on 8 december 2011 the acoustic temporal pattern of this event was consistent with other recorded large transient events in the literature and the major event was correlated with dramatic changes in other measurements including increased water column fluid velocities increased pressure and decreased salinity indicating real changes in emission flux observed seabed morphology changes reported elsewhere in this special issue also likely were related to this event these data demonstrate the dynamic nature of marine seepage systems show the value of monitoring systems and provide direct supporting evidence for a violent formation mechanism of many widespread seep associated seabed features like pockmarks

characteristic variations of sea surface temperature with multiple time scales in the north pacific abstract temporal evolution and spectral structure of sea surface temperature sst anomalies in the north pacific over the last 37 years are investigated on the three characteristic time scales shorter than 24 months hf 24 60 months es and longer than 60 months dc the leading empirical orthogonal function eof for the dc time scale is characterized by a zonally elongated monopole centered at around 40 n 180 the leading eof for the hf time scale is somewhat similar to that for the dc time scale although there are two centers of action with the same polarity at the mid and western pacific the leading eof for the es time scale however exhibits a different pattern whose center of action at the mid pacific is located farther southeastward in the time evolution of the sst anomalies associated with the leading eof of the dc time scale several anomaly periods can be identified that last five years or longer the transition from a persistent period to another with the opposite polarity is gene

observations of backscatter from sand and gravel seafloors between 170 and 250 khz interpreting observations of frequency dependence in backscatter from the seafloor offers many challenges either because multiple frequencies are used for different observations that will later be merged or simply because seafloor scattering models are not well understood above 100 khz hindering the understanding of these observations is the paucity of reported calibrated acoustic measurements above 100 khz this manuscript seeks to help elucidate the linkages between seafloor properties and frequency dependent seafloor backscatter by describing observations of backscatter collected from sand gravel and bedrock seafloors at frequencies between 170 and 250 khz and at a grazing angle of 45 overall the frequency dependence appeared weak for all seafloor types with a slight increase in seafloor scattering strength with increasing frequency for an area with unimodal very poorly to moderately well sorted slightly granular to granular medium sand with significant amounts of shell debris and a slight decrease in all other locations

the buoyancy and wind driven ventilated thermocline abstract an analytical study of the combined wind driven and buoyancy driven thermocline problem is presented the analysis is an extension of the ventilated thermocline model of luyten et al an exact solution for the ventilated region of the subtropical gyre is found under the condition that the ratio of the buoyancy flux to ekman pumping is a function of latitude alone it demonstrates the general westward shift of streamlines when the subtropical gyre is heated and the enlargement of the eastern shadow zone the flow in the shadow zone is described in most instances for the case of relatively small buoyancy flux it is shown that the shadow zone now in motion due to buoyancy forcing splits into two zones there is a broad eastward zone in which the deep flow is northeastward and a narrow zone between this branch and the ventilated fluid in which the shadow zone flow is returned in a relatively swift current nearly preserving potential vorticity for buoyancy ekman flux ratios which are nearly inde

initiation and displacement of landslide induced by earthquake a study of shaking table model slope test landslide researches often focus on factor of safety critical acceleration and displacement for an earthquake induced landslide the initiation and displacement of the landslide were important factors for definition of failure in this research the shaking table model slope test is adopted for study of slope failure initiation based on analyses of limit equilibrium particle image velocimetry piv analysis and acceleration time history recorded within the model slope based on the analyses landslide initiation could be defined as surface and local movement of the slope slope failure could be defined based on the massive movement of the slope the piv analysis provides good information for identifying both surface movement and massive movement the landslide displacement determined from the slope movement provides consistent results compared to observed displacements

topographical and geological amplifications determined from strong motion and aftershock records of the 3 march 1985 chile earthquake site response experiments were performed 5 months after the ms 7 8 central chile earthquake of 3 march 1985 to identify amplification due to topography and geology topographical amplification at canal beagle a subdivision of vina del mar was hypothesized immediately after the main event when extensive damage was observed on the ridges of canal beagle using frequency dependent spectral ratios of aftershock data obtained from a temporarily established dense array it is shown that there is substantial amplification of motions at the ridges of canal beagle the data set constitutes the first such set depicting topographical amplification at a heavily populated region and correlates well with the damage distribution observed during the main event dense arrays established in vina del mar also yielded extensive data which are quantified to show that in the range of frequencies of engineering interest there was substantial amplification at different sites of different geological formations to substantiate this spectral ratios developed from the strong motion records of the main event are used to show the extensive degree of amplification at an alluvial site as compared to a rock site similarly spectral ratios developed from aftershocks recorded at comparable stations qualitatively confirm that the frequency ranges for which the amplification of motions occur are quite similar to those from strong motion records in case of weak motions the denser arrays established temporarily as described herein can be used to identify the frequency ranges for which amplification occurs to quantify the degree of frequency dependent amplification and used in microzonation of closely spaced localities

acoustic estimates of methane gas flux from the seabed in a 6000 km2 region in the northern gulf of mexico seeps of free methane gas escaping the seabed can be found throughout the ocean basins to understand the role of methane gas seeps in the global carbon cycle including both gas added to the atmosphere and that which is dissolved and potentially oxidized in the ocean volume it is important to quantify the amount of methane escaping the seabed few large scale mapping projects of natural methane seeps have been undertaken however and even among these quantitative estimates of flux are rare here we use acoustic mapping techniques to survey 357 natural methane seeps in a large region 6000 km2 of the northern gulf of mexico and outline a general approach for methane seep mapping using a combination of multibeam and split beam echo sounders using additional measurements collected with a remotely operated vehicle rov together with the acoustic mapping results we estimate the total gas flux within the 6000 km2 region to be between 0 0013 and 0 16 tg yr or between 0 003 and 0 3 of the current estimates for global seabed methane seepage rates

evidences of landslide earthquake triggering due to self excitation process the basin like setting of stiff bedrock combined with pre existing landslide masses can contribute to seismic amplifications in a wide frequency range 0 10 hz and induce a self excitation process responsible for earthquake triggered landsliding here the self excitation process is proposed to justify the far field seismic trigger of the cerda landslide sicily italy which was reactivated by the 6th september 2002 palermo earthquake ms 5 4 about 50 km far from the epicentre the landslide caused damage to farm houses roads and aqueducts close to the village of cerda and involved about 40 106 m3 of clay shales the first ground cracks due to the landslide movement formed about 30 min after the main shock a stress strain dynamic numerical modelling performed by fdm code flac 5 0 supports the notion that the combination of local geological setting and earthquake frequency content played a fundamental role in the landslide reactivation since accelerometric records of the triggering event are not available dynamic equivalent inputs have been used for the numerical modelling these inputs can be regarded as representative for the local ground shaking having a pga value up to 0 2 m s2 which is the maximum expected in 475 years according to the italian seismic hazard maps a 2d numerical modelling of the seismic wave propagation in the cerda landslide area was also performed it pointed out amplification effects due to both the structural setting of the stiff bedrock at about 1 hz and the pre existing landslide mass in the range 3 6 hz the frequency peaks of the resulting amplification functions a f fit well the h v spectral ratios from ambient noise and the h h spectral ratios to a reference station from earthquake records obtained by in situ velocimetric measurements moreover the fourier spectra of earthquake accelerometric records whose source and magnitude are consistent with the triggering event show a main peak at about 1 hz this frequency value well fits the one amplified by the geological setting of the bedrock in correspondence with the landslide area which is constituted of marly limestones and characterised by a basin like geometry

on boundary value problems of the ideal fluid thermocline abstract recent developments of ideal fluid thermocline models are briefly reviewed using density coordinates boundary value problems are formulated for the ideal fluid thermocline with continuous stratification ekman pumping and surface density are specified as the upper boundary conditions no flow is permitted through the ocean s eastern boundary nor its bottom each water column is divided into three parts i e the stagnant abyssal water with specified stratification the unventilated thermocline with its potential vorticity specified and the ventilated thermocline with its potential vorticity determined by a global dynamic balance the unventilated thermocline is further divided into the shallow and deep parts potential vorticity is specified a priori for the latter however for the former potential vorticity has to be chosen in the process of calculating the solution so as to make the solution self consistent numerical integration of the ideal fluid thermocline equations is reduced to repea

a numerical investigation of the subduction process in response to an abrupt intensification of the westerlies abstract a three dimensional ocean general circulation model forced by idealized zonal winds is used to investigate the effect of an abrupt intensification of westerly winds on the subduction process four experiments are carried out 1 a control experiment with standard wind stress forcing 2 an intensified winds experiment with wind stress that is larger in the region of the westerlies than the control 3 an increased surface cooling experiment and 4 an experiment with both intensified wind stress and surface cooling experiments 2 through 4 which contain surface anomalous forcing are run from the steady state obtained in experiment 1 the control experiment the results obtained for each of these runs are compared to the control experiment a subarctic tracer injection experiment is also carried out to verify the differences in the subduction process of each of these experiments in the wind stress intensified experiment an examination of the subsurface temperature field shows that negative t

topographic effects on the seismic response of steep slopes a frequency domain parametric study using generalized consistent transmitting boundaries has been performed to evaluate the significance of topographic effects on the seismic response of steep slopes the results show that the peak amplification of motion at the crest of a slope occurs at a normalized frequency h 0 2 where h is the slope height and is the wavelength of the motion the importance of the natural site frequency is illustrated by the analysis of a stepped layer over a half space it was found that the natural frequency of the region behind the crest can dominate the response relative to the topographic effect for the conditions studied moreover the effect of topography can be handled separately from the amplification due to the natural frequency of the deposit behind the crest of the slope this concept of separating the amplification caused by topography from that caused by the natural frequency is advantageous to the development of a simplified method to estimate topographic effects

modelling the erosional susceptibility of landslide catchments in thick loess chinese variations on a theme by jan de ploey abstract in his 1990 paper catena 17 175 183 jan de ploey proposed a system whereby catchments are viewed as functional units in which geomorphic work results from the combined effects of water erosion and mass movements his measure of erosional susceptibility of catchments es introduced the gravitational potential in addition to the kinetic energy of the eroding agents this modelling of the erosional susceptibility of catchments in terms of energy was being used to explore time dependent variations of es when jan de ploey died also at this time he extended an invitation to the present authors to test and extend his model using our data base on the landslide dominated catchments in the thick loess country of north central china this paper presents a series of new equations expressing variations in es using a data base for over 200 landslides in the loess covered mountainous terrain relative relief up to 1500 m of the lanzhou region of eastern gansu province these equations express the loss of potential energy of the landslide mass in relation to the relative relief and the pressure energy input in the form of rainfall these indices of erosional susceptibility can be used to analyse the differences in response from one catchment to another and through time between the characteristics of vegetation cover including land use hydrology geomechanical behaviour and mechanisms of movement on slopes using certain assumptions on percentage area of catchments affected by mass movement and long term mean annual precipitation in the lanzhou region the es equation suggests an estimate for non clayey materials of about 10 4 m 2 s 2 which is a significantly higher susceptibility value than the mean worldwide value for non clayey materials obtained by de ploey

interdecadal thermocline variability in the north pacific for 1958 97 a gcm simulation an ocean general circulation model is forced with the ncep reanalysis wind stress for 1958 97 to understand mechanisms of ocean subsurface variability with relatively high horizontal 1 83 18 and vertical 41 levels resolutions the model produces mode waters on a range of density surfaces in the western central and eastern north pacific in qualitative agreement with observations these mode waters appear as a thermostad or a region of weak stratification in the upper thermocline as they flow southward from their formation regions in the kuroshio and its extension in the model subsurface temperature variability in the central subtropical gyre reaches a maximum within the thermostad in contrast to what might be expected from the linear baroclinic rossby wave theory this variance maximum is associated with the longitudinal shift in the path of mode waters in particular deepened mixed layer and accelerated eastward currents in the kuroshio extension by wind changes in the mid 1970s act cooperatively to move the central mode waters toward the east causing large subsurface temperature anomalies besides the local maximum in the central north pacific subtropical gyre two additional maxima of the subsurface anomaly are identified in the northwestern and southern parts of the gyre respectively among these subsurface anomaly centers the one in the northwestern north pacific has a strong effect on the model sea surface temperature suggesting that the kuroshio and its extension are a key region to decadal interdecadal ocean atmosphere interaction finally possible effects of atmospheric thermal forcing are discussed

the application of finite element analysis to body wave propagation problems summary the finite element method is shown to be a powerful tool for the numerical modelling of seismic body wave propagation problems applications extend to both problems on a scale of interest to engineers and also to large scale seismological problems solutions are sought in the time domain efficient programs have been written to accomplish this the scope of numerical solutions has been greatly enhanced by the use of a previously reported scheme for exactly cancelling reflections at the boundaries of the model the finite difference results of boore and the analytical results of trifunac for the amplification due to a mountain and an alluvial valley respectively are compared with new finite element results the new results agree well although there are some difficulties with resonance in the alluvial valley problem boore s sh results have been extended to vertical p and sv incidence a deep earthquake zone has been modelled realistically in two dimensions and earthquakes simulated at depth it is suggested that the variation in observed amplitude across the top of the zone due to refraction away from the slab may be used to provide an estimate of the thickness of the slab from long period observations of local earthquakes

seismically induced rock slope failures resulting from topographic amplification of strong ground motions the case of pacoima canyon california abstract the 1994 northridge earthquake mw 6 7 triggered extensive rock slope failures in pacoima canyon immediately north of los angeles california pacoima canyon is a narrow and steep canyon incised in gneissic and granitic rocks peak accelerations of nearly 1 6 g were recorded at a ridge that forms the left abutment of pacoima dam peak accelerations at the bottom of the canyon were less than 0 5 g suggesting the occurrence of topographic amplification topographic effects have been previously suggested to explain similarly high ground motions at the site during the 1971 mw 6 7 san fernando earthquake furthermore high landslide concentrations observed in the area have been attributed to unusually strong ground motions rather than higher susceptibility to sliding compared with nearby zones we conducted field investigations and slope stability back analyses to confirm the impact of topographic amplification on the triggering of landslides during the 1994 earthquake our results suggest that the observed extensive rock sliding and falling would have not been possible under unamplified seismic conditions which would have generated a significantly lower number of areas affected by landslides in contrast modelling slope stability using amplified ground shaking predicts slope failure distributions matching what occurred in 1994 this observation confirms a significant role for topographic amplification on the triggering of landslides at the site and emphasises the need to select carefully the inputs for seismic slope stability analyses

groundwater and earthquakes screening analysis for slope stability groundwater is a factor behind the occurrence of landslides and so is the action of seismic shaking in sloping terrain this paper presents closed form expressions for the factor of safety and yield coefficient of slopes subjected simultaneously to seismic forces and variable groundwater conditions two failure modes of natural slopes are considered in this work namely long slope with phreatic surface parallel to the ground surface and slope with emerging phreatic surface for these failure modes the factor of safety and yield coefficient are determined for various conditions of drainage using effective stress analysis and total stress analysis the effect of soil strength reduction by seismic loading is accounted for in the derived factors of safety and yield coefficients the specification of the lateral seismic coefficient for equivalent seismic loading relies on recent advances in geotechnical earthquake engineering and seismic engineering geology the role of liquefaction and clay softening is introduced in the analysis of seismic slope stability and slope deformation considering variable groundwater conditions several examples illustrate the application of the methods herein presented

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

ground motion observations at hotel montana during the m 7 0 2010 haiti earthquake topography or soil amplification unusually severe structural damage was reported during the 2010 m 7 0 haiti earthquake in the vicinity of hotel montana located on top of a ridge in the district of petionville prompted by the observations u s geological survey seismic stations were deployed and aftershock recordings indicated ground motion amplification on the top of the hill compared to adjacent stations on reference site conditions the presence of topographic relief has been shown to significantly aggravate the consequences of strong ground motion during past events and topographic effects were brought forward to explain the observations in this paper we test the hypothesis of topographic amplification as the dominant factor that contributed to the damage concentration in the vicinity of hotel montana we initially conduct numerical simulations of the ridge seismic response assuming elastic homogeneous site conditions and show that numerical predictions of topographic amplification disagree with the field data both in amplitude and in frequency conversely while 1d ground response analyses for the site conditions at the hilltop predict amplification in the same frequency range as the field data they significantly underestimate the recorded amplitude we then conduct numerical simulations of the foothill ridge response to seismic motion while accounting for soil layering and qualitatively demonstrate that the recorded amplification is most likely attributed to coupled site topographic amplification effects namely to seismic waves trapped in the soft soil layers of the near surface amplified as a consequence of reverberations and further modified due to diffraction and scattering upon incidence on the irregular ground surface parametric investigations of the topography soil amplification coupling effects are then conducted and our results show that when accounting for a hypothetical soil bedrock interface at 100 m depth predictions are in excellent agreement with the observed motion

on the dynamics of the coupled mixed layer thermocline system and the determination of the oceanic surface density abstract a simple model of the oceanic mixed layer is coupled to a model of the ventilated thermocline the model allows a combination of advection and surface heating to determine the position of the outcrop lines of the isopycnals the resulting isopycnal outcrops determine the circulation in the ventilated thermocline as in the 1983 study by luyten pedlosky and stommel lps the isopycnal outcrop line is affected by both ekman wind drift and the surface geostrophic flow hence the outcrop position and the thermocline circulation am coupled the mixed layer and the thermocline models are extremely simple each is modeled by layers of constant density the mixed layer in which the isopycnals are vertical is distinguished by the ability of fluid to cross the interfaces between adjacent layers under the influence of atmospheric heating the heating is parameterized in terms of the departure of the isopycnal line from the position it would have if the ocean were heated but at rest although in most ma

application of a characteristic periods based cpb approach to estimate earthquake induced displacements of landslides through dynamic numerical modelling the authors would like to thank the european union erdf for financial support via the monitorizacion sismica de deslizamientos criterios de reactivacion y alerta temprana project of the programa operativo feder de andalucia 2007 2013

a statistics based discrete element modeling method coupled with the strength reduction method for the stability analysis of jointed rock slopes abstract the stability of jointed rock slopes can be effectively analyzed using the discrete element method dem however the criteria for terminating the dem iterative calculation process and determining the factor of safety fos are not well defined in this paper a displacement statistics based discrete element modeling method dsdm is proposed to analyze the stability of jointed rock slopes by coupling with the shear strength reduction method srm in the presented dsdm two important criteria are defined 1 a statistics based criterion for terminating the iterative calculation process of the dem modeling in the stability analysis of jointed rock slopes and 2 a statistics based criterion for determining the fos of a jointed rock slope when coupling dem and srm the novelty of the proposed dsdm is that statistical information of the global displacements around the potential slip surface of the jointed slope rather than the local displacements of several characteristic points are utilized to define the criteria for terminating the dem iterative calculation process and determining the fos to verify the effectiveness of the proposed dsdm the fos of a simplified jointed rock slope is first calculated using the proposed dsdm and then compared with the fos obtained using the finite difference method fdm moreover the proposed dsdm is applied to analyze the stability of a real high steep bedding rock slope the numerical simulation results for the real rock slope are consistent with the in situ observations the proposed dsdm is thus capable of effectively analyzing the stability of jointed slopes with potential slip surfaces

seismic performance of loess mudstone slope in tianshui centrifuge model tests and numerical analysis abstract dynamic behavior and failure mechanism of slope during earthquake are hot topics in geotechnical engineering community in tianshui area northwest of china the deformation features of loess mudstone slopes induced by earthquakes are still not well understood the current study aims to discuss the dynamic behavior of the slope under earthquake by performing a series of centrifuge shaking table tests and numerical simulations the results are processed for pga amplificatory effect study and to evaluate the earthquake induced deformation mechanism better agreement is obtained between the results of numerical simulations and centrifuge tests the results indicated that the acceleration response of the loess mudstone slope showed several amplifying characteristic the height effect lithologic effect and surface effect it also proves that the displacements in the loess layer are much larger than mudstone as the input amplitude increased the shear plastic zone developed along a further slide surface tension plastic zone mainly occurred at the crest roughly the regions of earthquake induced deformation state increased with the increase in amplitude of earthquake wave deformations occurred at the upper part of the slope tensile cracks evolved on the crest and the upper part of slope in loess layer moreover failures occurred at slope surface and only slight deformation at slope toe deformations with continuous shear cracks and slope surface upheavals developed at the middle part of slope

geological and geomorphological precursors of the chiu fen erh shan landslide triggered by the chi chi earthquake in central taiwan abstract special features were correlated to the geological causes of the chiu fen erh shan landslide a gigantic rockslide on a dip slope induced by the chi chi earthquake ml 7 3 in central taiwan in 1999 an aerial photo interpretation and the succeeding geological mapping of the failure were employed in this study a linear depression a steep step and a low drainage density in the landslide area were detected from the aerial photos taken in 1998 the gravitational creep was believed to result in the features of the linear depression and the low drainage density the steep step represented a buckling feature found in the field the landslide area is composed of stratified sandstone and shale with dip angles ranging 20 36 the slip surface developed along a pre existing bedding fault that resulted from flexural slip folding before the chi chi earthquake the rock on the upslope side buckled and was retained by a thick bedded sandstone downslope the earthquake shock seriously damaged the sandstone support and led to the catastrophic landslide this type of landslide is likely to occur on the moderately dipping slope of stratified rocks that were previously deformed by flexural slip folding

the role of finite mixed layer thickness in the structure of the ventilated thermocline abstract a model of the ventilated thermocline consisting of three adiabatic layers surmounted by a mixed layer of finite thickness is presented the mixed layer depth density increase continuously northward and these attributes of the mixed layer are specified the effect of the mixed layer on the thermocline circulation is explicitly calculated the mixed layer thickness and its variation play a significant role in shifting the trajectories of the streamlines westward the shadow zones enlarge more rapidly south of the outcrop lines the finite mixed layer depth and its increase northward produce shadow zones in each of the adiabatic layers of the thermocline the augmentation of the ventilation rate i e the rate at which fluid enters the thermocline from the mixed layer in excess of the ekman pumping is directly proportional to the northward gradient of the mixed layer potential vorticity the enhancement is greatest in regions where the circulation is surface intensified i e above the eastern sh

the role of the mixed layer in setting the potential vorticity of the main thermocline abstract a steady ventilation model is used to assess the effect of the mixed layer on the structure of the main thermocline the potential vorticity is found in a subtropical gyre after imposing the thickness and density of the mixed layer the ekman pumping and the hydrography on the eastern boundary the modeled potential vorticity becomes comparable in value to observations in the north atlantic if the mixed layer deepens poleward as is observed in winter the isopycnal gradients in potential vorticity are reduced on the denser ventilated surfaces if the mixed layer outcrops deviate from latitude circles and more realistically sweep southward along the eastern boundary the age of the subducted fluid is also in reasonable agreement with observations of the tritium helium age by jenkins this study suggests that ventilation may form much of the extensive region of nearly uniform potential vorticity observed on the 26 75 surface in the north atlantic with lateral mixing by eddies being required

the interaction of seismic waves with step like slopes and its influence on landslide movements abstract the interaction of seismic waves with slopes is a major factor influencing landslide movements that involve slope stability local site seismic amplification and topographic effects affecting ground motion the results of a numerical study of landslide movements induced by the interaction of seismic waves with step like slopes are presented here to investigate this input slope interaction a dynamic analysis was performed using the finite difference stress strain numerical code flac 6 0 under visco plastic conditions the dynamic signals were selected to be representative of different peak ground accelerations pgas arias intensities and frequency contents and they were used in a parametric study of different step like slopes with different geometrical configurations in terms of dip height and thickness of geological strata the derived outputs were processed for a seismic amplification analysis and to evaluate the induced stress strain effects in terms of progressive failure and resulting displacements the obtained results i describe a fundamental role of topography in amplifying or de amplifying the seismic ground motion ii demonstrate that the progressive failure of unsheared slopes influences the seismic amplification iii show that the strain effects on unsheared slopes in terms of progressive failure are more intense with increasing arias intensity and slope dip iv prove that amplification or de amplification processes can justify the values of displacements involving pre existing landslide masses which are significantly different with respect to those expected on the basis of sliding block approaches i e newmark s and flexible sliding block methods v highlight that in the geological setting considered here the seismically induced displacements arising from the reactivation of pre existing landslide masses can be significantly underestimated by sliding block approaches in the case of low angle slopes characterised by high k values i e the ratios between the critical pseudostatic threshold ky of the landslide and pgas of the applied seismic input

ground motions at the outermost limits of seismically triggered landslides abstract over the last few decades we and our colleagues have conducted field investigations in which we mapped the outermost limits of triggered landslides in four earthquakes 1987 whittier narrows california m 5 9 1987 superstition hills california m 6 5 1994 northridge california m 6 7 and 2011 mineral virginia m 5 8 in an additional two earthquakes 1976 guatemala m 7 5 and 1983 coalinga california m 6 5 we determined limits using high resolution aerial photographic interpretation in conjunction with more limited ground investigation limits in these earthquakes were defined by the locations of the very smallest failures 3 from the most susceptible slopes that can be identified positively as having been triggered by earthquake shaking because we and our colleagues conducted all of these investigations consistent methodology and criteria were used in determining limits in the six earthquakes examined we correlated the outermost landslide limits with peak ground accelerations pgas from shakemap models of each earthquake for the four earthquakes studied by field investigation the minimum pga values associated with farthest landslide limits ranged from 0 02 g to 0 08 g the range for the two earthquakes investigated using aerial photographic interpretations was 0 05 0 11 g although pga values at landslide limits depend on several factors including material strength topographic amplification and hydrologic conditions these values provide an empirically useful lower limiting range of pga needed to trigger the smallest failures on very susceptible slopes in a well recorded earthquake this pga range can be used to identify an outer boundary within which we might expect to find landsliding in earthquakes that are not well recorded mapping the outermost landslide limits provides a useful clue about ground motion levels at the mapped limits

on the seismic response of deep seated rock slope instabilities insights from numerical modeling abstract earthquake induced landslides constitute a critical component of seismic hazard in mountainous regions while many seismic slope stability analysis methods exist with varying degrees of complexity details of interactions between seismic waves and incipient landslides are not well understood and rarely incorporated in particular for deep seated slope instabilities we present a series of 2d distinct element numerical models aimed at clarifying interactions between earthquakes and large rock slope instabilities the study has two main goals 1 to explore the role of amplification in enhancing co seismic slope deformation a relationship widely discussed in literature but rarely tested quantitatively and 2 to compare our numerical results with the well established newmark method which is commonly used in seismic slope stability analysis we focus on three amplification phenomena 1 geometric topographic amplification 2 amplification related to material contrasts and 3 amplification related to compliant fractures slope height topography seismic velocity contrasts and internal strength and damage history were varied systematically in a series of models with a relatively simple scalable geometry for each model we compute the spatial amplification patterns and displacement induced by real earthquake ground motions we find that material contrasts and internal fracturing create both the largest amplification factors and induced displacements while the effect of geometry is comparably small newmark type sliding block methods underestimate displacements by not accounting for material contrasts and internal fracturing within the landslide body both common phenomena in deep seated slope instabilities although larger amplification factors tend to be associated with greater displacements we did not identify a clear link between ground motion frequency content spectral amplification and induced displacement nevertheless observation of amplification patterns can play an important role in seismic slope stability analyses as 1 strong amplification related to material contrasts or compliant fractures is an indicator of potentially large co seismic displacements and 2 amplification patterns can be used to constrain geological and numerical models used for seismic stability analysis the complexity of wave slope interactions as well as the potential to severely underestimate hazard using newmark type methods motivates use of rigorous numerical modeling for quantitative seismic hazard and risk assessment of large landslides

a numerical investigation of effects of a tilt of the zero wind stress curl line on the subduction process a three dimensional ocean general circulation model is used to investigate the effects of an idealized nonzonal wind stress curl on the subduction process in the subtropical gyre idealized zonal winds are used to force the model ocean analyses of potential vorticity water particle trajectories and tracer injection experiments are conducted two characteristic features appear in response to the nonzonal distribution of the wind stress curl an upwelling region occurs in the subtropical gyre and a downwelling region extends northward in the northeastern corner of the subtropical gyre in comparison to results obtained from a zonal wind stress curl a pool region where there is no signal from the surface of the subarctic region shrinks a ventilated region becomes deeper and the subducted water has a wider range of potential density

upper ocean thermal variations in the north pacific during 1970 1991 abstract a newly available extensive compilation of upper ocean temperature profiles was used to study the vertical structure of thermal anomalies between the surface and 400 m depth in the north pacific during 1970 1991 a prominent decade long perturbation in climate occurred during this time period surface waters cooled by 1 c in the central and western north pacific and warmed by about the same amount along the west coast of north america from late 1976 to 1988 comparison with data from coads suggests that the relatively sparse sampling of the subsurface data is adequate for describing the climate anomaly the vertical structure of seasonal thermal anomalies in the central north pacific shows a series of cold pulses beginning in the fall of 1976 and continuing until late 1988 that appear to originate at the surface and descend with time into the main thermocline to at least 400 m depth individual cold events descend rapidly 100 m yr 1 superimposed upon a slower cooling 15 m yr 1 the inter

rotational translational landslides in the neogene basins at the northeast margin of the tibetan plateau abstract rotational translational landslides are common in neogene basins throughout the world and are of high risk to the public to understand the mechanism of rotational translational failure the spatial distribution and deformation of rotational translational landslides in the northeast of the tibetan plateau are investigated in this study the spatial distribution of these landslides is dependent on the regional tectonics and geomorphology crustal stresses and lithological properties the rotational translational landslides are concentrated in the neogene mudstone basins and the intensities of these landslides are observed to gradually decrease from the hinterland of the plateau to the marginal basins the bedding parallel shear zones within the location of the rotational translational landslides are present in the overconsolidated neogene mudstones with high clay content nearly horizontal tectonic stresses and erosion cause the formation of horizontal shear stresses in the sliding masses in this stress environment materials with low internal friction angle

thermocline forced by annual and decadal surface temperature variation abstract a two layer thermocline model is modified by adding an essentially passive mixed layer above it the surface temperature variation is simulated by a moving outcrop line it is found that in contrast to a surface wind stress a surface temperature variation causes strong variability in the ventilated zone through subducted water while it affects the shadow zone little two types of buoyancy forced solution are found when the outcrop line moves slowly the solutions are nonentrainment solutions for these solutions the surface beat flux is mainly balanced by the horizontal advection in the permanent thermocline the mixed layer never entrains the time mean thermocline is close to the steady thermocline with the time mean outcrop line when the outcrop line moves southward rapidly during the cooling season the solutions become entrainment solutions now deep vertical convection must occur because the horizontal advection in the permanent thermocline is no longer strong enough to balance the s

a numerical model of the ventilated thermocline abstract a steady state numerical solution is found for an idealized rectangular ocean basin driven by wind and surface buoyancy flux a three dimensional primitive equation model is used in agreement with recent analytical modeling the thermocline in the numerical solution consists of three regions quite distinct in their ventilation characteristics forming the greater part of the subtropical thermocline is an unventilated pool zone located in the core of the subtropical gyre and a ventilated zone to the east the unventilated shadow zone lies farther east and toward the equator analysis of potential vorticity on constant density surfaces is used to study the structure of the thermocline a small but intense zone of convection located in the western boundary outflow caused by rapid heat loss to the atmosphere produces source water for the ventilated zone this water of extremely low potential vorticity mode water is distributed widely into the subtropical thermocline the pool forms equ

centrifuge model test on the retrogressive landslide subjected to reservoir water level fluctuation abstract geological disasters have occurred frequently in the three gorges reservoir area therefore the stability and failure mode research on riverside landslides has become increasingly important taking the toe of liangshuijing landslide as a case study and combining the geological conditions of the three gorges reservoir area a centrifuge slope model is constructed in addition the fluctuation of the water level is simulated in the centrifuge model test based on the digital photography laser displacement sensors lds pore pressure transducers ppt and earth pressure sensors e multi physical data are obtained finally numerical simulation is carried out to analyze the stability and deformation characteristics of the slope model the results indicate that 1 at the water level rise stage the model is dominated by the vertical displacement and the horizontal displacement is much larger than the vertical displacement at the drainage stage 2 cracks first appear at the toe of the slope and extend to the middle and rear of the model gradually then deformation occurs in the middle platform of the slope 3 at the water level rise stage the factor of safety increases rapidly when the reservoir water level remains stable the stability of the slope decreases slowly and at the drainage stage the factor of safety decreases first and then increases 4 there is a significant positive linear correlation between the decline rate of the safety factor and the drainage rate over a certain test range and the stability of the landslides on the reservoir banks would become worse gradually during the long term cyclic operation of the three gorges reservoir

thermocline forced by varying ekman pumping part i spinup and spindown abstract a two layer planetary geostrophic model is used to investigate the thermocline variability under a suddenly changing ekman pumping the effect of ventilation and the associated advection is particularly emphasized in the ventilated zone the governing equation is a quasi linear equation which is solved analytically by the method of characteristics it is found that the dynamics differs substantially between a shadow zone and a ventilated zone in the shadow zone the rossby wave is the dominant mechanism to balance the ekman pumping after a sudden change in the wind field the ekman pumping changes rapidly but the baroclinic rossby wave evolves at a much slower time scale years to decades this mismatch of response time scale produces an imbalance in forcings and in turn results in a strong thermocline variability however in the ventilated zone the cold advection replaces the rossby wave to become the major opposing mechanism to the ekman pumping after a sudden wind change both the ekma

widespread landslides induced by the mw 5 1 earthquake of 11 may 2011 in lorca se spain abstract on 11 may 2011 a moderate magnitude mw 5 1 earthquake struck the city of lorca se spain and nearby areas causing notable damage injuries and fatalities field inspection of the area affected allowed the location and characterization of more than 250 induced landslides mainly of disrupted type rock soil falls disrupted soil slides and rock avalanches this number was very high when compared with the inventories of previous events of similar magnitude mw 4 7 to 5 0 in the zone and with the results of previous models for predicting slope behavior during earthquakes several factors are proposed here to explain the observed differences i the comparatively high magnitude mw 4 5 of a foreshock of the seismic series that may have weakened the slopes ii the high peak ground motion values recorded for these events and iii the occurrence of topographic site effects the results of this study point out the importance of including such effects as improvements in the models for studying the regional behavior of slopes

a case study of site effects on seismic ground motions at xishan park ridge in zigong sichuan china abstract the city of zigong faces severe seismic hazards because of the amplifying effects of its topography and geology to estimate the local site effects an array of 8 accelerographs has been in operation along the xishan park ridge since 2007 in this study site amplifications associated with the ground motion produced by the 2008 wenchuan earthquake is evaluated using the standard spectral ratio ssr technique and root mean square acceleration arms method 2d numerical analysis using finite difference method is then performed and results are compared with monitoring data the results show that topography and local presence of soft materials significantly increase the ground motion in both field observations and the numerical model along the ridge slope the maximum ground motion amplification was obtained at the hilltop for a frequency range 1 10 hz for both ns and ew components significant amplifications occurred at higher frequencies 10 hz and are considered to result from local specific conditions such as rock fracturing and ridge steps thus they do not necessarily occur at the hilltop a slight difference between the two horizontal components ns and ew was observed and this is possibly due to the azimuthal dependence of site effect the computed amplification ratios give amplitudes that are comparable to those indicated by field observations however the calculated spectral amplifications are underestimated at frequencies larger than 3 hz this can be due to the fact that a signal damping ratio used for the whole model does not adequately represent the energy dissipation at high frequency

impact of tensile strength and incident angles on a soil slope under earthquake sv waves abstract the tensile strength and incident angles play important roles in seismically induced slope failures in this paper a new criterion for tension shear coupling is proposed based on several sets of relevant lab data and is verified using the experimental data provided by previous researchers this criterion can perfectly depict the tension region as well as the compression region in the entire stress space moreover the tension shear criterion was converted into an expression that could be directly used in a numerical method and it was verified by a numerical example in addition the sv waves were input using the equivalent nodal force method based on a viscous spring artificial boundary this process was implemented by the abaqus software and two numerical examples were used to verify this method subsequently based on the proposed new criterion and input method the impacts of the strength criterion and incident angles on an analysis of seismically induced slope failures were studied the results indicated that the evaluation of the tension shear region is extremely important in the analysis of the seismically induced slope failures the tensile strength of the seismically induced landslides was overestimated by the calculation of mohr coulomb criterion owing to the ambiguous description of the tensile failure or the tension shear failure the assessment of the slope stability was thus not conservative the incident angles of seismic waves affected the travel path and the directivity of slope seismic responses the sliding displacement of the slope side near the epicenter rose with increasing angles of wave incidence the critical slip surface deepened and the hazard aggravated therefore it is important to introduce both the tensile strength and incident angles when evaluating the stability of slopes subjected to seismic loads

landslides triggered by excavation in the loess plateau of china a case study of middle pleistocene loess slopes abstract the frequent occurrence of catastrophic landslides caused by excavation in a loess slope is one of the commonly encountered engineering problems in the loess plateau middle pleistocene loess slopes were selected as a case study for current research the distribution of excavation induced loess landslides was studied by field investigation on the basis of centrifuge model tests the deformation features and failure mechanism of a homogeneous loess slope and a double paleosol interlayer loess slope under different excavation slope angles were analyzed and compared it was shown that the response of horizontal earth pressures in the lateral region of the excavation zone has a close relationship with the failure of the slope displacement measurements showed that the deformation stages of loess slopes due to excavation are independent of the paleosol interlayer the excavation slope inclinations and soil properties significantly affected the failure process and division of the slope the characteristics that excavation induced loess landslides always occur in hysteresis and abruptness were interpreted by the analysis of the failure process the influence of blade penetration during the excavation performed was quantitatively estimated finally based on the result of stress path tests an improved unloading constitutive model was proposed to simulate the deformation of soils in the lateral and bottom of the excavation area

the effect of topography on earthquake ground motion a review and new results a brief review of experimental and theoretical results about the effect of topography on seismic motion shows that they are consistent only on a qualitative basis amplification at mountain tops for wavelengths comparable with mountain widths is predicted and observed but the numerical simulations often underestimate the actual observed effects we propose that this disagreement is because current models are not complex enough we therefore computed the response to incident sh waves of a set of different complex configurations that include two dimensional surface topography with or without periodic ridges and subsurface layering the results show that 1 the topographic effect in itself is difficult to isolate from other effects like surface layering and therefore the amplification on top of geomorphologically complex sites cannot be predicted by a priori estimations based solely on topography and 2 the high crest base amplification ratio observed in the field cannot usually be matched even with complex two dimensional structures with incident plane sh waves which suggests that more complex models are needed to incorporate more complex wave fields e g sv surface and three dimensional geologic configurations

influence of lixiviation by irrigation water on residual shear strength of weathered red mudstone in northwest china implication for its role in landslides reactivation abstract irrigation in loess tableland areas of northwest china has induced a large number of landslides around their edges of which loess red mudstone landslide is one of the most frequent types salt sinters commonly observed at the toes of these landslides where the weathered red mudstone wrm outcrops have raised the possibility that removal of soluble salts in the wrm by irrigation water could be crucial in the reduction of its shear strength and thus in the occurrence of these landslides in order to investigate the effects of lixiviation by irrigation water on residual shear strength of the wrm samples of the wrm close to two landslides in lanzhou city one of the largest cities in northwest china were tested after being treated under different conditions these included 1 saturation with the yellow river water the only source of irrigation 2 saturation with groundwater and 3 saturation and leaching with deionized water based on the results of these tests specific mechanisms of residual shear strength reduction of the wrm due to lixiviation by irrigation water were discussed in terms of physico chemical interactions between the wrm and water it was found that lixiviation reduced residual shear strength and residual friction angle r of the wrm by up to 65 and 62 respectively after being leached 6 times this coincided with a dramatic dissolution and leaching of soluble salts from the wrm particle size distribution showed that the more the wrm was leached the more the clay fraction and the lesser coarser fractions this suggested that reduction in residual shear strength of the wrm could be attributed to mainly disaggregation of particles coarser than clay and to a lesser degree to weakening of interparticle forces among individual clays it was further postulated that variation in clay mineralogy may influence performance of particle disaggregation

eastern north pacific subtropical mode water in a general circulation model formation mechanism and salinity effects the eastern north pacific subtropical mode water estmw is a water mass of low potential vorticity pv and appears as a weak pycnostad or thermostad distinct from other subtropical mode waters it forms in the absence of a deep winter mixed layer the formation mechanism of this estmw is investigated using an ocean general circulation model that is forced by monthly climatological temperature salinity and wind stress at the sea surface an equation based on the ventilated thermocline theory is used to diagnose the initial pv of a water mass right after its subduction in this equation three factors affect the initial pv the spacing of density outcrop lines the mixed layer depth gradient and the vertical velocity at the bottom of mixed layer among them the wide spacing between outcrop lines is the most important for estmw s low pv instead of the deep mixed layer which is most important for classical mode waters it is found that weak gradients in both sea surface temperature and salinity in the direction of mixed layer flow are important for the low pv formation a low salinity tongue that extends southeastward off north america is responsible for the small surface density gradient in the eastern north pacific and contributes to the formation of the estmw an additional experiment forced with observed freshwater flux demonstrates that the southward advection of fresher water from the high latitude along the eastern boundary is the cause of this low salinity tongue

north pacific premonitory sign of the enso event 1 the premonitory sign of the enso event shows up in the north pacific subtropical mode water stmw 18 months earlier and this should be useful in forecasting the occurrence of this important phenomenon the premonitory connection involves an air sea coupling between the longtime persistent mid latitude ssta induced by the remote re emergence of stmw and the following spring subtropical atmospheric circulation anomalies examination of the air sea interaction reveals that the following spring subtropical atmospheric circulation which responds to the longtime persistent ssta is dominated by the anomalous negative positive geopotential height downstream of the negative positive ssta in the strong weak stmw case then the tropics adjust to these anomalies through coupled dynamics producing enso like pattern of variability details of the connection and possible process are presented

split beam echo sounder observations of natural methane seep variability in the northern gulf of mexico a method for positioning and characterizing plumes of bubbles from marine gas seeps using an 18 khz scientific split beam echo sounder sbes was developed and applied to acoustic observations of plumes of presumed methane gas bubbles originating at approximately 1400 m depth in the northern gulf of mexico a total of 161 plume observations from 27 repeat surveys were grouped by proximity into 35 clusters of gas vent positions on the seafloor profiles of acoustic target strength per vertical meter of plume height were calculated with compensation for both the sbes beam pattern and the geometry of plume ensonification these profiles were used as indicators of the relative fluxes and fates of gas bubbles acoustically observable at 18 khz and showed significant variability between repeat observations at time intervals of 1 h 7 5 months active gas venting was observed during approximately one third of the survey passes at each cluster while gas flux is not estimated directly in this study owing to lack of bubble size distribution data repeat surveys at active seep sites showed variations in acoustic response that suggest relative changes in gas flux of up to 1 order of magnitude over time scales of hours the minimum depths of acoustic plume observations at 18 khz averaged 875 m and frequently coincided with increased amplitudes of acoustic returns in layers of biological scatterers suggesting acoustic masking of the gas bubble plumes in these layers minimum plume depth estimates were limited by the sbes field of view in only five instances

gyres driven by combined wind and buoyancy flux abstract the combined effects of buoyancy forcing and wind in a gyre scale steady ocean circulation are modeled using discrete layers with an interfacial flux not necessarily vertical the equations for vorticity conservation of the geostrophic flow in this system are fully nonlinear involving a jacobian for the layer thicknesses these equations are written in a form which can be solved by the method of characteristics the form of these equations invites the interpretation that the geostrophic baroclinic flow is driven by buoyancy and steered by the wind two examples are solved and discussed a subtropical gyre with heating and a subpolar gyre with cooling in each case there are distinct regimes of flow depending upon whether the characteristics originate at the eastern or western boundaries of the model a simple geometrical argument illustrates that the difference between these two regimes the direct and indirect cells depends upon the sign of the true vertical velocity relative to the interf

subtropical countercurrent in an idealized ocean gcm abstract in the north pacific there is a shallow eastward current called the subtropical countercurrent which flows across the central subtropical gyre the present article studies the generation mechanism of the subtropical countercurrent reproduced in an ocean general circulation model gcm with a simple geometry driven by surface wind stress and surface buoyancy forcing in the ocean gcm the deep mixed layer occurs in the northern part of subtropical gyre and shoals abruptly in the central subtropical gyre the mixed layer front the narrow transition zone of the mixed layer depth slants from the western central subtropical gyre to the northeast and the low potential vorticity fluid is formed at the intersection of the mixed layer front and the outcrop line since the surface density is almost zonally uniform and the mixed layer front slants northeastward the minimum potential vorticity fluids on denser isopycnals are formed in the northeastern region while those on lighter isopycnals are form

simple model prediction of horizontal temperature fields in the subtropical subpolar system caused by sudden change in wind stress curl abstract a four level numerical model which is driven by wind stress and surface heat flux is used to study responses of horizontal temperature fields in the subtropical subpolar system to a sudden change in the magnitude of the wind stress curl weakly nonlinear responses to o 1 change in the wind stress curl are examined according to the effects on the ekman pumping convection westward baroclinic wave and advection for this purpose a quasi analytical method that is characteristics associated with effects of both wave propagation and advection is constructed based on a planetary geostrophic model with four level geometry characteristics obtained for the first and second baroclinic modes are used to diagnose steady state and time dependent solutions one feature of time dependent motions predicted by the model is a westward propagation of the first baroclinic mode with a significantly higher speed than the combined speed of the nondispersive first mode baroclinic rossby wave and barotropic ci