inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

southern ocean isopycnal mixing and ventilation changes driven by winds observed and predicted changes in the strength of the westerly winds blowing over the southern ocean have motivated a number of studies on the response of the antarctic circumpolar current and southern ocean meridional overturning circulation moc to wind perturbations and led to the hypothesis of the eddy compensation regime wherein the moc becomes insensitive to wind changes in addition to the moc tracer transport also depends on mixing processes here we show in a high resolution process model that isopycnal mixing by mesoscale eddies is strongly dependent on the wind strength this dependence can be explained by mixing length theory and is driven by increases in eddy kinetic energy the mixing length does not change strongly in our simulation simulation of a passive ventilation tracer analogous to cfcs or anthropogenic co2 demonstrates that variations in tracer uptake across experiments are dominated by changes in isopycnal mixing rather than changes in the moc we argue that to properly understand tracer uptake under different wind forcing scenarios the sensitivity of isopycnal mixing to winds must be accounted for

the role of mesoscale eddies in the rectification of the southern ocean response to climate change abstract simulations from a fine resolution global coupled model the geophysical fluid dynamics laboratory climate model version 2 4 cm2 4 are presented and the results are compared with a coarse version of the same coupled model cm2 1 under idealized climate change scenarios a particular focus is given to the dynamical response of the southern ocean and the role played by the eddies parameterized or permitted in setting the residual circulation and meridional density structure compared to the case in which eddies are parameterized and consistent with recent observational and idealized modeling studies the eddy permitting integrations of cm2 4 show that eddy activity is greatly energized with increasing mechanical and buoyancy forcings buffering the ocean to atmospheric changes and the magnitude of the residual oceanic circulation response is thus greatly reduced although compensation is far from being perfect changes in poleward eddy fluxes partially compensate for the enhanced equatorward

high stress drop events in the victoria baja california earthquake swarm of 1978 march summary the victoria earthquake swarm of 1978 march with magnitudes up to ml 4 8 occurred near the northern end of the cerro prieto fault in northern baja california mexico accurate epicentre locations of a number of earthquakes in the swarm reveal that the activity concentrated in a zone of about 6 km radius projected on the horizontal with earthquakes occuring mostly at depths of around 12 km a composite fault plane solution prepared with data from the larger earthquakes of the swarm indicates right lateral strike slip motion along a vertical plane extending parallel to the cerro prieto fault seismic moment source radius and stress drop are calculated from strong motion records and digital seismograph records obtained at short epicentral distances in most cases less than 10 km from calculated displacement spectra and brune s model stress drops between 1 bar and 1 kbar were estimated for earthquakes with seismic moments in the range 1 4 1019 2 9 1023 dyne cm eighteen events of 44 studied here had stress drops higher than 90 bar a result in striking contrast to lower stress drop results from previous distant station studies of earthquakes in the region results of comparing near source and large distance spectral shapes for two earthquakes in the victoria swarm region indicate that scattering and attenuation are a likely cause of the severe reduction of high frequency energy clearly shown by the larger distance spectra and thus of the resulting lower stress drop determinations at large distances if these events are characteristic of other large events in the region then the low stress drop determinations of thatcher and thatcher hanks for this region may be in doubt very high peak horizontal ground accelerations up to 0 6 g were measured from the near source records which were characterized by short duration bursts of high frequency energy it is suggested here that these high peak accelerations resulted from a combined effect of high stress drop and sediment amplification using a crustal model derived from mcmechan mooney a sedimentary amplification factor of 3 4 is estimated and used as a correction in calculation of the earthquake source parameters four other high stress drop up to 2 5 kbar earthquakes in this area were also analysed the high stress drop values obtained from seismic spectra are corroborated by using the rms root mean square acceleration formulation introduced by hanks

global ocean meridional overturning abstract a decade mean global ocean circulation is estimated using inverse techniques incorporating air sea fluxes of heat and freshwater recent hydrographic sections and direct current measurements this information is used to determine mass heat freshwater and other chemical transports and to constrain boundary currents and dense overflows the 18 boxes defined by these sections are divided into 45 isopycnal neutral density layers diapycnal transfers within the boxes are allowed representing advective fluxes and mixing processes air sea fluxes at the surface produce transfers between outcropping layers the model obtains a global overturning circulation consistent with the various observations revealing two global scale meridional circulation cells an upper cell with sinking in the arctic and subarctic regions and upwelling in the southern ocean and a lower cell with sinking around the antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges

empirical green s functions from small earthquakes a waveform study of locally recorded aftershocks of the 1971 san fernando earthquake seismograms from 52 aftershocks of the 1971 san fernando earthquake recorded at 25 stations distributed across the san fernando valley are examined to identify empirical green s functions and characterize the dependence of their waveforms on moment focal mechanism source and recording site spatial variations recording site geology and recorded frequency band recording distances ranged from 3 0 to 33 0 km hypocentral separations ranged from 0 22 to 28 4 km and recording site separations ranged from 0 185 to 24 2 km the recording site geologies are diorite gneiss marine and nonmarine sediments and alluvium of varying thicknesses waveforms of events with moment below about 1 5 1021 dyn cm are independent of the source time function and are termed empirical green s functions waveforms recorded at a particular station from events located within 1 0 to 3 0 km of each other depending upon site geology with very similar focal mechanism solutions are nearly identical for frequencies up to 10 hz there is no correlation to waveforms between recording sites at least 1 2 km apart and waveforms are clearly distinctive for two sites 0 185 km apart the geologic conditions of the recording site dominate the character of empirical green s functions even for source spatial separations of up to 20 0 km the empirical green s functions at a particular site are consistent in frequency content amplification and energy distribution therefore it is shown that empirical green s functions can be used to obtain site response functions the observations of empirical green s functions are used as a basis for developing the theory for using empirical green s functions in deconvolution for source pulses and synthesis of seismograms of larger earthquakes

a source extent analysis of the imperial valley earthquake of october 15 1979 and the victoria earthquake of june 9 1980 a new method for estimating source extent parameters has been applied to the imperial valley earthquake of october 15 1979 and the victoria earthquake of june 9 1980 from the stations of the world wide standard seismograph network and canadian seismic network estimates of the duration of far field long period sh waves were made by computing the variance or second central moment 2 of instrument corrected attenuation corrected pulses seventeen measurements for the imperial valley event and 25 for the victoria event were made we inverted for the source duration t the fault length l and the directivity parameter d by least squares we find that for the imperial valley t 9 0 0 5 s d 130 43 km s and l 46 13 km and for the victoria event t 10 9 0 6 s and d 251 71 km s l is not well constrained for this second event the inversion gives 23 km the lower bound obtained by constraining 2 to be positive everywhere on the focal sphere although a value as high as 40 km would still be within the uncertainties the positive values of d for both events indicate that rupture propagation was predominantly to the northwest in order to estimate the degree of unilateralness we have computed the rupture mode index defined by r d lt which is 1 for a unilateral rupture and zero for a bilateral rupture we find a value of r 0 31 0 14 for the imperial valley event significantly less than unity which indicates that a southern component of rupture is also present assuming uniform slip we estimate the northern and southern fault segments to be 26 and 20 km respectively the presence of a bilateral component is supported by near field studies which have made use of stations along the southern half of the imperial fault for the victoria event due primarily to the uncertainty in l the value of r can vary from 0 6 to 1 with the value of 1 being most probable assuming that the rupture is unilateral we can obtain an estimate of fault length from the relation l d t which yields l 23 6 km geodetic data and the distribution of aftershocks are consistent with a unilateral rupture mode and the fault length that we have obtained values of seismic moment from measurements of the zeroth moment of the displacement pulse are found to be 5 8 0 4 for imperial valley and 4 8 0 4 for victoria in units of 1018 n m the presence of a southern component of rupture for the imperial valley event would suggest a closer and perhaps causal relationship to the victoria event which occurred 8 months later and about 60 km to the south in a tectonic framework we interpret these two earthquakes as the failure of adjacent transform faults and consider the possibility that a concurrent spreading event may have occurred in the vicinity of cerro prieto

overturning circulation driven by breaking internal waves in the deep ocean a global estimate of the water mass transformation by internal wave driven mixing in the deep ocean is presented the estimate is based on the energy conversion from tidal and geostrophic motions into internal waves combined with a turbulent mixing parameterization we show that internal wave driven mixing in the deep ocean can sustain 20 30 sv of water mass transformation one third or more of this transformation is attributed to lee waves generated by geostrophic motions flowing over rough topography primarily in the southern ocean while these results are uncertain due to many assumptions poorly constrained parameters and data noise that enter in the calculation the result that lee wave driven mixing plays an important role in the abyssal ocean circulation is likely robust the implication is that lee wave driven mixing should be represented in ocean and climate models but currently it is not citation nikurashin m and r ferrari 2013 overturning circulation driven by breaking internal waves in the deep ocean geophys res lett 40 doi 10 1002 grl 50542

antarctic sea ice control on the depth of north atlantic deep water abstractchanges in deep ocean circulation and stratification have been argued to contribute to climatic shifts between glacial and interglacial climates by affecting the atmospheric carbon dioxide

a δ13c record of upper north atlantic deep water during the past 2 6 million years benthic foraminiferal 813c data from site 502 in the caribbean sea sill depth 1800 m indicate that throughout the past 2 6 m y glacial 8 3c values in the middepth atlantic were higher during glaciations than interglaciations this is interpreted as indicating a greater proportion of upper north atlantic deep water unadw relative to southern source waters during glaciations the contribution of unadw during interglaciations to the middepth atlantic remained approximately constant and the contribution during glaciations may have been as much as 10 higher in the late pleistocene than in the late pliocene this small increase is in striking contrast to the much larger decrease in glacial lower north atlantic deep water lnadw contribution relative to southern sources from about 80 to about 20 that occurred over the past 2 6 m y glacial intensification over the past 2 6 m y was probably coupled with a decrease in northward heat transport by the upper limb of the north atlantic circulation cell as was previously suggested on the basis of a lnadw record alone late pleistocene 1 ma present 813c values in the caribbean sea were approximately 0 2 0 higher than they were from 2 6 to 2 0 ma the 813c rise is not due to an increase in the mean ocean 8 3c value nor can it be entirely attributed to an increase in the proportion of high 8 3c source waters an increase in the 813c value of the surface source waters must have contributed to the 8 3c rise

the seismic spectrum radiated energy and the savage and wood inequality for complex earthquakes abstract we have integrated velocity squared spectra in order to determine the seismic energy radiated during fault rupture the high frequency spectral fall off and the shape of the spectrum at the corner frequency are critical to the energy calculation high frequency spectral fall offs of 2 beyond the corner frequency in a brune 1970 source model return radiated energies approximately equal to that of an orowan 1960 type fault failure where the final stress level is equal to the dynamic frictional stress any spectra with an extended intermediate slope of 1 would therefore result in higher radiated energies savage and wood 1971 proposed a model in which the final stress level was less than the dynamic stress level and that this was the result of overshoot they based their model on the observation that the ratio of twice the apparent stress to the stress drop was typically around 0 3 we show that for such a ratio to exist high frequency spectral fall offs of 3 would be required composite spectra have been constructed for several moderate to large earthquakes these spectra have been compared to that predicted by the haskell 1966 model and velocity squared spectra have been integrated to determine the radiated energy in all cases this ratio twice the apparent stress to the stress drop is greater than or equal to one violating the savage and wood 1971 inequality and provides evidence against overshoot as a source model

source mechanism and surface wave excitation for two earthquakes in northern baja california mexico summary two earthquakes of comparable local magnitude from different regions of northern baja california the 1975 july 17 pino solo earthquake ml 5 1 from the peninsular ranges region and the 1976 december 7 mesa de andrade earthquake ml 5 3 from the colorado river delta region are studied in detail to determine possible causes for the observed stronger excitation of surface waves by earthquakes from the delta region relative to earthquakes of comparable local magnitude in the peninsular ranges region data from distant stations are complemented with data from aftershock studies using local arrays and for the mesa de andrade earthquake with data from two strong motion stations the strong motion records for ihe mesa de andrade earthquake show that it consisted of two events 45 s apart which produced maximum recorded horizontal accelerations of 0 21 and 0 24g respectively at a distance of 18 km from the epicentre synthetic seismograms are used to help determine the depth of these events and their source time functions both events are relatively simple the second event is sharp and impulsive and could be thought of as an aftershock since it has a considerably smaller moment than the first event even though its recorded acceleration was higher comparison of measured parameters shows that while the local magnitude of the mesa de andrade earthquake is only 0 2 units larger than the pino solo earthquake and its source dimension is 2 2 times larger source areas 4 8 times larger its moment is larger by a factor of 6 2 8 5 this is approximately explained by the cy2 scaling law however the near source spectra and accelerations recorded on the strong motion accelerograph at riito at a distance of about 18 km are considerably larger than predicted by the scaling law this is probably a results of high stress drop asperities on the fault surface although near field focusing directivity may also have contributed

sensitivity of the overturning circulation in the southern ocean to decadal changes in wind forcing abstractthe sensitivity of the overturning circulation in the southern ocean to the recent decadal strengthening of the overlying winds is being discussed intensely with some works attributing an inferred saturation of the southern ocean co2 sink to an intensification of the overturning circulation while others have argued that this circulation is insensitive to changes in winds fundamental to reconciling these diverse views is to understand properly the role of eddies in counteracting the directly wind forced changes in overturning here the authors use novel theoretical considerations and fine resolution ocean models to develop a new scaling for the sensitivity of eddy induced mixing to changes in winds and they demonstrate that changes in southern ocean overturning in response to recent and future changes in wind stress forcing are likely to be substantial even in the presence of a decadally varying eddy field this result has significant implications for the ocean s role in the carbon cycle and

oceanic isopycnal mixing by coordinate rotation abstract current numerical models of oceanic circulation differentiate between the eddy diffusion and viscosity transport along the geopotential horizontal and vertical directions only in order to model the effect of anisotropic turbulence as diffusive transport along and across density surfaces the isopycnal mixing tensor has been transformed from a diagonal second rank tensor in the isopycnal coordinate system to a tensor containing off diagonal elements in the geopotential coordinate system

four views of a portion of the north atlantic subtropical gyre abstract data from four cruises to the triangle centered at 27 n 32 30 w were smoothed by fitting second degree polynomials at each of about 30 different density surfaces the density intervals were p 0 3 corresponding to about 50 db intervals in pressure from the polynomials determination was made of central values horizontal derivatives and laplacians of the fields of pressure salt oxygen and dynamic height in addition maps of the fits and deviations of each nation from the smoothed fits were produced from the steady advective diffusive equation and the smoothed fits to the age oxygen and dynamic height fields the lateral isopycnal diffusivity as estimated to be kh 0 5 103 m2 s 1 although the salt field was reasonably stable from cruise to cruise the variability of the baroclinic velocity shear was found to be as large as the baroclinic shear itself the maps suggest a wobble of the gyre the standard deviation of the fluctuations at each nation from the smoothed fits when norma

overturning circulation pathways in a two basin ocean model abstracta toy model for the deep ocean overturning circulation in multiple basins is presented and applied to study the role of buoyancy forcing and basin geometry in the ocean s global overturning

interbasin transport of the meridional overturning circulation abstractthe meridional overturning circulation moc is studied in an idealized domain with two basins connected by a circumpolar channel in the southernmost region flow is forced at the surface by longitude independent wind stress freshwater flux and fast temperature relaxation to prescribed profiles the only longitudinal asymmetry is that one basin is twice as wide as the other two states a preferred one with sinking in the narrow basin and an asymmetrically forced one with sinking in the wide basin are compared in both cases sinking is compensated by upwelling everywhere else including the passive basin despite the greater area of the wide basin the residual overturning transport is the same regardless of the location of sinking the two basins exchange flow at their southern edge by a geostrophic transport balanced by the difference in the depth of isopycnals at the eastern boundaries of each basin gnanadesikan s model for the upper branch of the moc is extended to include two basins conn

ventilation of the deep ocean constrained with tracer observations and implications for radiocarbon estimates of ideal mean age abstract ocean ventilation is the process that transports water and climatically important trace gases such as carbon dioxide from the surface mixed layer into the ocean interior quantifying the dominant source regions and time scales remains a major challenge in oceanography a mathematically rigorous approach that accounts for the multiplicity of transport pathways and transit times characteristic of an eddy diffusive flow such as the ocean is to quantify ventilation in terms of a probability distribution that partitions fluid parcels according to the time and location of their last surface contact here we use globally gridded radiocarbon data in combination with other transient cfcs and hydrographic temperature salinity phosphate and oxygen tracer data to estimate the joint distribution of age and surface origin of deep ocean waters our results show that 40 and 26 of the global ocean was last in contact with the southern ocean and north atlantic respectively some 80 of the global deep ocean below 1500 m is ventilated from these high latitude regions however contrary to the classical description of the deep ocean as a roughly equal mixture of northern and southern source waters we find a significantly higher contribution from the southern ocean relative to the north atlantic we estimate the mean transit time from the surface to the deep north pacific at 1360 350 y intermediate between two widely used radiocarbon based estimates to reconcile our estimate of the ideal mean age with ventilation age estimates based on radiocarbon we apply the estimated distribution function to construct a 3 dimensional distribution of the water mass fraction weighted surface initial radiocarbon concentration that can serve as an accurate reservoir age radiocarbon ages corrected for this initial reservoir age are found to be in good agreement within 5 with our ideal age estimate demonstrating that it is essential to take into account the spatially variable surface radiocarbon field when computing ventilation ages using radiocarbon a wide spectrum of ages contributes to the mean age providing evidence for the fundamentally eddy diffusive nature of the large scale general circulation of the ocean

the age of water and ventilation timescales in a global ocean model abstract the age of water in the world ocean is studied using a passive age tracer introduced into a global ocean model additional information is derived from a transient dye tracer that tracks the time dependent spreading of surface waters into the model ocean interior of particular interest is the nature of ocean ventilation over the 10 100 yr timescale as well as the simulated age of deep and bottom water masses in the upper model levels young water is found to correspond with regions of convergence and downwelling in the surface ekman layer upwelling and convection are both shown to age the upper ocean by entraining older waters into the surface mixed layer in the deep model levels water age varies greatly between oceans with young water found in convectively active regions in the north atlantic and in the ross and weddell seas and old water found in the deep north pacific the oldest water mass mixture located at 2228 m depth in the western pacific ocean is dated at 1494 years made

transit time and tracer age distributions in geophysical flows transport in the atmosphere and in the ocean is the result of the complex action of time dependent and often highly turbulent flow a useful diagnostic that summarizes the rate at which fluid elements are transported from some region to a point or the reverse via a multiplicity of pathways and mechanisms is the probability density function pdf of transit times the first moment of this pdf often referred to as mean age has become an important transport diagnostic commonly used by the observational community this paper explores how to probe the flow with passive tracers to extract transit time pdf s as a foundation the literal tracer age is defined as the elapsed time since tracer was injected into the flow and the corresponding tracer age distribution z as the fractional tracer mass in a given interval of tracer age the distribution z has concrete physical interpretation for arbitrary sources but is only equivalent to a tracer independent transit time pdf of the flow in special cases the transit time pdf is a propagator g9 of boundary conditions the age spectrum of t m hall and r a plumb applied over a control surface v the propagator g9 is shown to be the flux into v resulting from a unit mass injected into the time reversed flow through explicit construction of the transit time pdf using the concept of tracer age the special cases for which z and g9 coincide are established this allows a direct physical demonstration of g9 and its adjoint g9 as the pdf s of transit times since fluid at point r had last contact with v and until fluid at r will have first contact with v respectively in the limit as v is shrunk to a point point to point transit time pdf s are well defined but their mean transit time and higher order moments become infinite several concrete geophysical examples are considered to illustrate under what conditions characteristics of tracer age and transit time pdf s can be inferred from observations in the atmosphere or the ocean

direct estimate of lateral eddy diffusivity upstream of drake passage abstractthe first direct estimate of the rate at which geostrophic turbulence mixes tracers across the antarctic circumpolar current is presented the estimate is computed from the spreading of a tracer released upstream of drake passage as part of the diapycnal and isopycnal mixing experiment in the southern ocean dimes the meridional eddy diffusivity a measure of the rate at which the area of the tracer spreads along an isopycnal across the antarctic circumpolar current is 710 260 m2 s 1 at 1500 m depth the estimate is based on an extrapolation of the tracer based diffusivity using output from numerical tracers released in a one twentieth of a degree model simulation of the circulation and turbulence in the drake passage region the model is shown to reproduce the observed spreading rate of the dimes tracer and suggests that the meridional eddy diffusivity is weak in the upper kilometer of the water column with values below 500 m2 s 1 and peaks at the steering level near 2 km where the eddy ph

absolute gravity measurements in california we have constructed an absolute gravity meter that determines the local gravitational acceleration by timing a freely falling mass with a laser interferometer the instrument has made measurements at 11 sites in california four in nevada and one in france the uncertainty in the results is typically 10 gal 1 gal 1 cm s2 1 gal 10 6 gal 10 8 m s2 repeated measurements have been made at several of the sites only one shows a substantial change in gravity

quantifying antarctic bottom water and north atlantic deep water volumes 1 a near global census of antarctic bottom water aabw and north atlantic deep water nadw is essayed through a nonnegative least squares analysis of conservative and quasi conservative seawater properties aabw thickness generally decreases from south to north modulated by ocean bathymetry likewise nadw thickness generally decreases from north to south in the atlantic ocean nadw dominates below the thermocline of the atlantic ocean at least as far south as the subtropical gyre of the south atlantic with a lesser but still significant influence around the entire antarctic circumpolar current in the indian ocean and in the pacific ocean however in the pacific and indian oceans aabw dominates below the thermocline in addition measurable quantities of aabw reach into the abyssal north atlantic on both sides of the mid atlantic ridge the census results suggest that aabw occupies roughly twice the volume of nadw in the three main oceans and that aabw is in contact with roughly twice the area of the deep main ocean floor compared with nadw however these results are somewhat sensitive to choices of water masses their values of seawater properties and the weightings of the seawater properties used in the analysis

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

mixing of a tracer in the pycnocline a patch of sulfur hexafluoride was released in may 1992 in the eastern north atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces the diapycnal eddy diffusivity k estimated for the first 6 months was 0 12 0 02 cm2 s while for the subsequent 24 months it was 0 17 0 02 cm2 s the vertical tracer distribution remained very close to gaussian for the full 30 months as the root mean square rms dispersion grew from 5 to 50 m lateral dispersion was measured on several scales the growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0 07 m2 s at scales of 0 1 to 1 km larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing teasing of the overall patch initially about 25 km across into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3 0 5 10 7 s 1 the rms width of these streaks estimated as 3 km and maintained in the face of the streak growth indicates an isopycnal diffusivity of 2 m2 s at scales of 1 to 10 km much greater than expected from internal wave shear dispersion the patch was painted in albeit streakily by 12 months confirming expectations from analytical and numerical models homogenization of the patch continued during the subsequent 18 months while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m2 s acting at scales of 30 to 300 km

simulations of strong ground motion for earthquakes in the mexicali imperial valley region summary in this paper computer modelling is used to test simple approximations for simulating strong ground motions for moderate and large earthquakes in the mexicali imperial valley region initially we represent an earthquake rupture process as a series of many independent small earthquakes distributed in a somewhat random manner in both space and time along the rupture surface by summing real seismograms for small earthquakes used as empirical green s functions strong ground motions at specific sites near a fault are calculated alternatively theoretical green s functions that include frequencies up to 20 hz are used in essentially similar simulations the model uses random numbers to emulate some of the non deterministic irregularities associated with real earthquakes due either to complexities in the rupture process itself and or strong variations in the material properties of the medium simulations of the 1980 june 9 victoria baja california earthquake ml 6 1 approximately agree with the duration of shaking the maximum ground acceleration and the frequency content of strong ground motion records obtained at distances of up to 35 km for this moderate earthquake in the initial stages of modelling we do not introduce any scaling of spectral shape with magnitude in order to see at what stage the data require it surprisingly such scaling is not critical in going from m 4 5 events to the m 6 1 victoria earthquake however it is clearly required by the el centro accelerogram for the imperial valley 1940 earthquake which had a much higher moment ms 7 we derive the spectral modification function for this event the resulting model for this magnitude 7 earthquake is then used to predict the ground motions at short distances from the fault predicted peak horizontal accelerations for the m 7 event are about 25 50 per cent higher than those observed for the m 6 1 victoria event

characterizing transport between the surface mixed layer and the ocean interior with a forward and adjoint global ocean transport model the theory of first passage time distribution functions and its extension to last passage time distribution functions are applied to the problem of tracking the movement of water masses to and from the surface mixed layer in a global ocean general circulation model the first passage time distribution function is used to determine in a probabilistic sense when and where a fluid element will make its first contact with the surface as a function of its position in the ocean interior the last passage time distribution is used to determine when and where a fluid element made its last contact with the surface a computationally efficient method is presented for recursively computing the first few moments of the first and last passage time distributions by directly inverting the forward and adjoint transport operator this approach allows integrated transport information to be obtained directly from the differential form of the transport operator without the need to perform lengthy multitracer time integration of the transport equations the method which relies on the stationarity of the transport operator is applied to the time averaged transport operator obtained from a three dimensional global ocean simulation performed with an ogcm with this approach the author i computes surface maps showing the fraction of the total ocean volume per unit area that ventilates at each point on the surface of the ocean ii partitions interior water masses based on their formation region at the surface and iii computes the three dimensional spatial distribution of the mean and standard deviation of the age distribution of water

eddy stirring and horizontal diffusivity from argo float observations geographic and depth variability stirring along isopycnals is a significant factor in determining the distribution of tracers within the ocean salinity anomalies on density surfaces from argo float profiles are used to investigate horizontal stirring and estimate eddy mixing lengths eddy mixing length and velocity fluctuations from the ecco2 global state estimate are used to estimate horizontal diffusivity at a 300 km scale in the upper 2000 m with near global coverage diffusivity varies by over two orders of magnitude with latitude longitude and depth in all basins diffusivity is elevated in zonal bands corresponding to strong current regions including western boundary current extension regions the antarctic circumpolar current and equatorial current systems the estimated mixing lengths and diffusivities provide an observationally based data set that can be used to test and constrain predictions and parameterizations of eddy stirring

a generalized transport theory water mass composition and age abstract a general theory to describe and understand advective and diffusive ocean transport is reported it allows any passive tracer field with an atmospheric source to be constructed by superposing sea surface contributions with a generalized green s function called the boundary propagator of the passive tracer equation the boundary propagator has the interpretation of the joint water mass and transit time distribution from the sea surface the theory thus includes the classical oceanographic idea of water mass analysis and extends it to allow for a distribution of transit times from the sea surface the joint water mass and transit time distribution contains complete information about the transport processes in the flow it captures this information in a more accessible way than using velocity and diffusivity fields however at least for the case of sequestration and transport of dissolved material by the ocean circulation the boundary propagator is thus the natural quantity to consider when discus

strong motion seismology strong motion seismology is concerned with earthquake ground motion in the amplitude range that poses the threat of human injury or property damage because the occurrence of strong motion cannot be predicted in advance and because the time between significant events at any one place may range from tens to thousands of years strong motion data is recorded on triggered instruments that typically must be maintained in readiness for years between periods of significant recording as a consequence the strong motion data set is sparse because of the engineering need for estimates of future ground motion the scope of the field of strong motion seismology encompasses seismic source theory and any other aspect of seismology that can provide insights helpful in making the best possible estimates given the limited data this relationship with the rest of seismology is a reciprocal one in that strong motion data gives perhaps the best information available on earthquake source processes

evidence for enhanced eddy mixing at middepth in the southern ocean abstract satellite altimetric observations of the ocean reveal surface pressure patterns in the core of the antarctic circumpolar current acc that propagate downstream eastward but slower than the mean surface current by about 25 the authors argue that these observations are suggestive of baroclinically unstable waves that have a steering level at a depth of about 1 km detailed linear stability calculations using a hydrographic atlas indeed reveal a steering level in the acc near the depth implied by the altimetric observations calculations using a nonlinear model forced by the mean shear and stratification observed close to the core of the acc coinciding with a position where mooring data and direct eddy flux measurements are available reveal a similar picture albeit with added details when eddy fluxes are allowed to adjust the mean state computed eddy kinetic energy and eddy stress are close to observed magnitudes with steering levels between 1 and 1 5 km broadly consistent with observatio

total matrix intercomparison a method for determining the geometry of water mass pathways abstract ocean tracer distributions have long been used to decompose the deep ocean into constituent water masses but previous inverse methods have generally been limited to just a few water masses that have been defined by a subjective choice of static property combinations through air sea interaction and upper ocean processes all surface locations are potential sources of distinct tracer properties and thus it is natural to define a distinct water type for each surface site here a new box inversion method is developed to explore the contributions of all surface locations to the ocean interior as well as the degree to which the observed tracer fields can be explained by a steady state circulation with unchanging surface boundary conditions the total matrix intercomparison tmi method is a novel way to invert observations to solve for the pathways connecting every surface point to every interior point in the limiting case that the circulation is steady and that five conservative tracers are perf

how much deep water is formed in the southern ocean three tracers are used to place constraints on the production rate of ventilated deep water in the southern ocean the distribution of the water mass tracer po4 phosphate star in the deep sea suggests that the amount of ventilated deep water produced in the southern ocean is equal to or greater than the outflow of north atlantic deep water from the atlantic radiocarbon distributions yield an export flux of water from the north atlantic which has averaged about 15 sv over the last several hundred years cfc inventories are used as a direct indicator of the current production rate of ventilated deep water in the southern ocean although coverage is as yet sparse it appears that the cfc inventory is not inconsistent with the deep water production rate required by the distributions of po4 and radiocarbon it has been widely accepted that the major part of the deep water production in the southern ocean takes place in the weddell sea however our estimate of the southern ocean ventilated deep water flux is in conflict with previous estimates of the flux of ventilated deep water from the weddell sea which lie in the range 1 5 sv possible reasons for this difference are discussed

a generalization of the stochastic summation scheme of small earthquakes to simulate strong ground motions a generalization of the stochastic summation scheme of small earthquakes following a general form of the source spectrum n model is developed to estimate the proper probability density functions for rupture propagation over the fault and to simulate site specific strong ground motions in two independent stages the probability density functions for rupture propagation are first developed for a single corner frequency source model and then extended to multiplicative and additive double corner frequency source models to partially account for the effects of finite fault rupture such as fault geometry rise time and rupture time on ground motions and to better match observed fourier amplitude spectra fas the generalized two stage stochastic summation scheme developed in this study can be used to simulate realistic strong ground motion time histories from small events represented as empirical green s functions egfs by specifying only two source parameters of the seismic moment and stress drop for a given site the proposed two stage stochastic summation schemes are tested using numerical examples following the theoretical source models as well as observed ground motions of the 2011 mw 9 0 tohoku earthquake in japan and the 1994 mw 6 7 northridge earthquake in california to verify the applicability of the approach comparisons between the observed and simulated time histories for these two earthquakes their response spectra and fas show satisfactory performance of the proposed approach for engineering purposes the results of these comparisons indicate that the proposed stochastic summation of small events as egfs can adequately replicate observed ground motions and evaluate uncertainties in ground motions with the generation of many time history realizations corresponding to a multitude of possible rupture processes the proposed approach can also be used to generate multiple realistic acceleration time histories matched on average to the target response spectra for use in earthquake performance design and to develop ground motion models for low seismicity regions where there is a lack of recording of strong ground motions

a 300 kyr record of aridity and wind strength in southwestern africa inferences from grain size distributions of sediments on walvis ridge se atlantic abstract the terrigenous fraction of sediments recovered from walvis ridge se atlantic ocean reveals a history of southwestern african climate of the last 300 kyr end member modelling of a data set of grain size distributions n 428 results in three end members the two coarsest end members are interpreted as eolian dust the third end member as hemipelagic mud the ratio of the two eolian end members reflects the eolian grain size and is attributed to the intensity of the se trade winds trade winds were intensified during glacials compared to interglacials changes in the ratio of the two eolian end members over the hemipelagic one are interpreted as variations in southwestern african aridity late quaternary southwestern african climate was relatively arid during the interglacial stages and relatively humid during the glacial stages owing to meridional shifts in the atmospheric circulation system during glacials the polar front shifted equatorward resulting in a northward displacement of the zone of westerlies causing increased rainfall in southwestern africa the equatorward shift of the polar front is coupled with an increase of the meridional pressure gradient leading to enhanced atmospheric circulation and increased trade wind intensity

global observations of horizontal mixing from argo float and surface drifter trajectories mixing by mesoscale eddies in the ocean plays a major role in setting the distribution of oceanic tracers with important implications for physical and biochemical systems at local to global scales roach et al 2016 https doi org 10 1002 2015jc011440 demonstrated that a two particle analysis of argo trajectories produces robust estimates of horizontal mixing in the southern ocean here we extend this analysis to produce global 1 1 maps of eddy diffusivity at the nominal argo parking depth of 1 000 m we also applied this methodology to estimate surface eddy diffusivities from global drifter program gdp surface drifters the global mean eddy diffusivity was 543 155 m2 s at 1 000 m and 2637 311 m2 s at the surface with elevated diffusivities in regions of enhanced eddy kinetic energy such as western boundary currents and along the antarctic circumpolar current the eddy kinetic energy at the equator is high at both the surface and depth but the eddy diffusivity is only enhanced near the surface at depth the eddy diffusivity is strongly suppressed due to the presence of mean flow we used our observational estimates to test the validity of an eddy diffusivity parameterization that accounts for mixing suppression in the presence of zonal mean flows our results indicated that this parameterization generally agrees with the directly observed eddy diffusivities in the midlatitude and high latitude oceans