adjoint tomography of the southern california crust using an inversion strategy based on adjoint methods we developed a three dimensional seismological model of the southern california crust the resulting model involved 16 tomographic iterations which required 6800 wavefield simulations and a total of 0 8 million central processing unit hours the new crustal model reveals strong heterogeneity including local changes of 30 with respect to the initial three dimensional model provided by the southern california earthquake center the model illuminates shallow features such as sedimentary basins and compositional contrasts across faults it also reveals crustal features at depth that aid in the tectonic reconstruction of southern california such as subduction captured oceanic crustal fragments the new model enables more realistic and accurate assessments of seismic hazard

past and future earthquakes on the san andreas fault the san andreas fault in calfornia is one of the best studied earthquake generating faults in the world in their perspective weldon et al discuss recent efforts to construct rupture scenarios that is possible histories of the date location and length of fault rupture of past earthquakes on the fault existing knowledge of the locations and dates of past earthquakes can be explained by different scenarios but the authors argue that the evidence favors the hypothesis that the fault breaks in relatively infrequent large earthquakes they caution that the current 148 year hiatus since the last big earthquake is likely to end within the next 50 years with a large rupture or sequence of ruptures spanning a substantial part of the fault

near bed turbulence and relict waveformed sand ripples observations from the inner shelf 1 results are presented from a field investigation of near bed turbulence above degrading waveformed sand ripples in 17 m water depth on the inner shelf the heights of the 50 cm wavelength primary ripples were about 5 cm at the start of the observation period and decreased by a factor of 2 within 15 days the principal degradation mechanism involved fish making pits in the seafloor near bed turbulent kinetic energy dissipation rates are estimated both from the energy spectrum and from the vertical structure function within the inertial subrange and ranged from 0 1 10 6 to 3 10 6 w kg the friction velocity u at the bed ranged from 0 3 to 0 5 cm s and the wave friction factor fw from 0 017 to 0 02 the nearbed turbulence intensities and consequently the estimated values of u2 and fw are likely too small by a factor of 2 partly to satisfy the smooth wall constraint and partly to account for the effects of small scale turbulence within the finite volume range cells of the coherent doppler system used to make the turbulence estimates finally the results indicate that the hydraulic roughness of relict ripples is likely a function of both ripple height and steepness and that the relative roughness should also depend on the near bed wave orbital excursion for modeling purposes nielsen s ripple roughness formula is recommended with a reduced proportionality constant to account for the effects of irregular wave forcing and non equilibrium ripple history

temporal and spatial evolution of wave induced ripple geometry regular versus irregular ripples concurrent observations of inner shelf near bed hydrodynamics and acoustic imagery of the seabed are used to relate wave induced ripple geometry wavelength and orientation to near bed directional wave velocities the observations were collected on the continental shelf of the south atlantic bight at water depths of 9 5 and 30 m off the coasts of south carolina median size 177 m and georgia 388 m respectively 2 d spectral analysis techniques are performed on the imagery to automate detection of ripple wavelength orientation and irregularity our analysis shows that ripple irregularity is a time dependent process dependent on magnitude direction and duration of wave forcing during energetic events ripple geometry changes rapidly and the ripples align with the main wave direction during periods of low energy close to the critical conditions for initiation of sediment motion ripple evolution occurs at a much slower rate often leading to irregularities such as terminations and bifurcations along the ripple crest under constantly changing wave direction the rippled bed becomes highly disorganized six types of ripples are defined based on newly developed irregularity parameters linear bifurcating linear linear bifurcating bifurcating and cross irregular and disorganized beds ripple irregularity depends on the time history of the bed ripples remain irregular until their wavelength and orientation attain a nearly equilibrium geometry the observations collected provide significant information on the response of the seabed to wave forcing and identify processes that should be reproduced by any time dependent ripple prediction model ripple irregularity can only be predicted using such time dependent models

turbulence measurements in the surf zone abstract velocity measurements within 1 m of the bottom in approximately 4 5 m water depth on a sand beach provide estimates of turbulent reynolds shear stress using a dual sensor technique that removes contamination by surface waves and inertial range estimates of dissipation when combined with wave measurements along a cross shore transect and nearby wind measurements the dataset provides direct estimates of the terms in simplified equations for alongshore momentum and turbulence energetics and permits examination of semiempirical relationships between bottom stress and near bottom velocity the records are dominated by three events when the measurement site was in the outer part of the surf zone near bottom turbulent shear stress is well correlated with squared correlation coefficient r2 0 63 but smaller than regression coefficient b 0 51 0 03 at 95 confidence wind stress minus cross shore gradient of wave induced radiation stress indicating that estimates of one or more of these ter

on the wave and current interaction with a rippled seabed in the coastal ocean bottom boundary layer interactions of currents and waves with a rippled seabed in the inner part of the coastal ocean bottom boundary layer are studied using particle image velocimetry adv and bottom roughness measurements mean velocity profiles collapse with appropriate scaling in the log layer but vary substantially in the roughness sublayer when wave induced motions are similar or greater than the mean current the hydrodynamic roughness z0 determined from velocity profiles is substantially larger than directly measured values the roughness signature in turbulent energy spectra persists with elevation when its scale falls in the dissipation range but decays in the log layer for larger roughness elements reynolds shear stress profiles peak in the lower parts of the log layer diminishing below it and gradually decaying at higher elevations in contrast wave shear stresses are negligible within the log layer but become significant within the roughness sublayer this phenomenon is caused by an increase in the magnitude and phase lag of the vertical component of wave induced motion no single boundary layer length scale collapses the reynolds stresses but both the prandtl mixing length and eddy viscosity profiles agree well with the classical model of linear increase with elevation especially near the seabed within the log region profiles of shear production and dissipation rates of turbulence converge below it dissipation rapidly increases peaking near the seabed conversely the shear production decays near the seabed in agreement with the eddy viscosity model but in contrast to both laboratory and computational rough wall studies

broadband simulations for mw 7 8 southern san andreas earthquakes ground motion sensitivity to rupture speed using the high performance computing resources of the southern california earthquake center we simulate broadband 0 10 hz ground motions for three m w 7 8 rupture scenarios of the southern san andreas fault the scenarios incorporate a kinematic rupture description with the average rupture speed along the large slip portions of the fault set at 0 96 0 89 and 0 84 times the local shear wave velocity consistent with previous simulations a southern hypocenter efficiently channels energy into the los angeles region along the string of basins south of the san gabriel mountains however we find the basin ground motion levels are quite sensitive to the prescribed rupture speed with peak ground velocities at some sites varying by over a factor of two for variations in average rupture speed of about 15 these results have important implications for estimating seismic hazards in southern california and emphasize the need for improved understanding of earthquake rupture processes

swell transformation across the continental shelf part i attenuation and directional broadening abstract extensive wave measurements were collected on the north carolina virginia continental shelf in the autumn of 1999 comparisons of observations and spectral refraction computations reveal strong cross shelf decay of energetic remotely generated swell with for one particular event a maximum reduction in wave energy of 93 near the virginia coastline where the shelf is widest these dramatic energy losses were observed in light wind conditions when dissipation in the surface boundary layer caused by wave breaking whitecaps was weak and wave propagation directions were onshore with little directional spreading these observations suggest that strong dissipation of wave energy takes place in the bottom boundary layer the inferred dissipation is weaker for smaller amplitude swells for the three swell events described here observations are reproduced well by numerical model hindcasts using a parameterization of wave friction over a movable sandy bed directional spectra that are narrow off the s

wind waves and moveable bed bottom friction abstract effects of moveable bed bottom friction for wave observations and wave modeling are investigated using a state of the art bottom friction model this model combines the hydrodynamic friction model of madsen et al with a moveable bed roughness model based on grant and madsen analyzing the present model for idealized swell cases it is shown that swell might result in wave generated sand ripples the large change of roughness corresponding to initial ripple formation results in a preferred wave height for swell related to bathymetric scales as generally occur in shelf seas away from the coast the corresponding wave generated bottom roughness is not defined by the local wave conditions but is related to the overall energy balance of the wave field sediment data thus is imperative for the interpretation of observed decay rates and friction factors for swell for idealized depth limited wind seas near bottom wave motion is expected to generate partially washed out ripples and moderate sheet flo

three dimensional simulation of a magnitude 7 75 earthquake on the san andreas fault simulation of 2 minutes of long period ground motion in the los angeles area with the use of a three dimensional finite difference method on a parallel supercomputer provides an estimate of the seismic hazard from a magnitude 7 75 earthquake along the 170 kilometer section of the san andreas fault between tejon pass and san bernardino maximum ground velocities are predicted to occur near the fault 2 5 meters per second and in the los angeles basin 1 4 meters per second where large amplitude surface waves prolong shaking for more than 60 seconds simulated spectral amplitudes for some regions within the los angeles basin are up to 10 times larger than those at sites outside the basin at similar distances from the san andreas fault

wave formed structures and paleoenvironmental reconstruction clifton h e and dingier j r 1984 wave formed structures and paleoenvironmental reconstruction in b greenwood and r a davis jr editors hydrodynamics and sedimentation in wave dominated coastal environments mar geol 60 165 198 wave formed sedimentary structures can be powerful interpretive tools because they reflect not only the velocity and direction of the oscillatory currents but also the length of the horizontal component of orbital motion and the presence of velocity asymmetry within the flow several of these aspects can be related through standard wave theories to combinations of wave dimensions and water depth that have definable natural limits for a particular grain size threshold of particle movement and that of conversion from a rippled to fiat bed indicate flow velocity limits the ratio of ripple spacing to grain size provides an estimate of the length of the near bottom orbital motion the degree of velocity asymmetry is related to the asymmetry of the bedforms though it presently cannot be estimated with confidence a plot of water depth versus wave height h h diagram provides a convenient approach for showing the combination of wave parameters and water depths capable of generating any particular structure in sand of a given grain size natural limits on wave height and inferences or assumptions regarding either water depth or wave period based on geologic evidence allow refinement of the paleoenvironmental reconstruction the assumptions and the degree of approximation involved in the different techniques impose significant constraints inferences based on wave formed structures are most reliable when they are drawn in the context of other evidence such as the association of sedimentary features or progradational sequences

adjoint analysis of the source and path sensitivities of basin guided waves summary simulations of earthquake rupture on the southern san andreas fault saf reveal large amplifications in the san gabriel and los angeles basins sgb and lab apparently associated with long range path effects geometrically similar excitation patterns can be recognized repeatedly in different saf simulations e g love wave like energy with predominant period around 4 s channelled southwestwardly from the sgb into lab yet the amplitudes with which these distinctive wavefield patterns are excited change depending upon source details slip distribution direction and velocity of rupture we describe a method for rapid calculation of the sensitivity of such predicted wavefield features to perturbations of the source kinematics using a time reversed adjoint wavefield simulation the calculations are analogous to those done in adjoint tomography and the same time reversed calculation also yields path sensitivity kernels that give further insight into the excitation mechanism for rupture on the southernmost 300 km of saf lab excitation is greatest for slip concentrated between the northern coachella valley and the transverse ranges propagating to the ne and with rupture velocities between 3250 and 3500 m s 1 along that fault segment that is within or slightly above the velocity range between rayleigh and s velocities that is energetically precluded in the limit of a sharp rupture front highlighting the potential value of imposing physical constraints such as from spontaneous rupture models on source parametrizations lab excitation is weak for rupture to the sw and for ruptures in either direction located north of the transverse transverse ranges whereas ventura basin vtb is preferentially excited by ne ruptures situated north of the transverse ranges path kernels show that lab excitation is mediated by surface waves deflected by the velocity contrast along the southern margin of the transverse ranges having most of their energy in basement rock until they impinge on the eastern edge of sgb through which they are then funnelled into lab vtb amplification is enhanced by a similar waveguide effect

scale model and numerical simulations of near fault seismic directivity foam rubber experiments simulating unilaterally propagating strike slip earthquakes provide a means to explore the sensitivity of near fault ground motions to rupture geometry subsurface accelerometers on the model fault plane show rupture propagation that approaches a limiting velocity close to the rayleigh velocity the slip velocity waveform at depth is cracklike slip duration of the order of narrower fault dimension w divided by s wave speed surface accelerometers record near fault ground motion enhanced along strike by rupture induced directivity most experimental features initiation time shape duration and absolute amplitude of acceleration pulses are successfully reproduced by a 3d spontaneous rupture numerical model of the experiments numerical and experimental model acceleration pulses show similar decay with distance away from the fault and fault normal components in both models show similar large amplitude growth with distance along fault strike this forward directivity effect is also evident in response spectra the fault normal spectral response peak at period w 3 increases approximately sixfold along strike on average in the experiments with similar increase about fivefold in the corresponding numerical simulation the experimental and numerical model response spectra agree with an empirical directivity model for natural earthquakes at long periods near w and both overpredict shorter period empirical directivity effects with the amount of overprediction increasing systematically with diminishing period we attribute this difference to rupture and wavefront incoherence in natural earthquakes due to fault zone heterogeneities in stress frictional resistance and elastic properties present in the earth but absent or minimal in the experimental and numerical models rupture front incoherence is an important component of source models for ground motion prediction but finding an effective kinematic parameterization may be challenging

the use of automated coal petrography in determining maceral group composition and the reflectance of vitrinite abstract using photometric and image analysis methods a reasonable technique and procedure have been developed for the determination of maceral group composition and the reflectance of vitrinite this procedure is introduced by an optimum method for seeking the interval values of each maceral group i e from the fingerprint reflectance histogram it seeks reasonable interval values and distribution values for vitrinite and other maceral groups after checking and correction these values can be put into a program and computerized this method had advantages such as increased speed of analysis and reproducibility of the results

geometry migration and evolution of wave orbital ripples at leo 15 observations of the temporal evolution of the geometric properties and migration of wave formed ripples are analyzed in terms of measured suspended sand profiles and water velocity measurements six weeks of bedform observations were taken at the sandy medium to coarse sized sand leo 15 site located on beach haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar during this period six tropical storms several of hurricane strength passed to the east of the study site ripples with wavelengths of up to 100 cm and with 15 cm amplitudes were observed the predominant ripples were found to be wave orbital scale ripples with ripple wavelengths equal to 3 4 of the wave orbital diameter although orbital diameters become larger than 130 cm during the maximum wave event it is unclear if a transition to non orbital scaling is occurring ripple migration is found to be directed primarily onshore at rates of up to 80 cm day suspended transport due to wave motions calculated by multiplying acoustic backscatter measurements of suspended sand concentrations by flow velocity measurements are unable to account for a sufficient amount of sand transport to force ripple migration and are in the opposite direction to ripple migration thus it is hypothesized that the onshore ripple migration is due to unobserved bedload transport or near bottom suspended transport bedload model calculations forced with measured wave velocities are able to predict the magnitude and direction of transport consistent with observed ripple migration rates sequences of ripple pattern temporal evolution are examined showing mechanisms for ripple directional change in response to changing wave direction as well as ripple wavelength adjustment and erosion due to changing wave orbital diameter and relative wave to current velocities

effects of large scale surface topography on ground motions as demonstrated by a study of the san gabriel mountains los angeles california we investigate the effects of large scale surface topography on ground motions generated by nearby faulting we show a specific example studying the effect of the san gabriel mountains which are bounded by the mojave segment of the san andreas fault on the north and by the los angeles basin on the south by si mulating a mw 7 5 earthquake on the mojave segment of the san andreas fault we show that the san gabriel mountains act as a natural seismic insulator for metropo litan los angeles the topography of the mountains scatters the surface waves gen erated by the rupture on the san andreas fault leading to less efficient excitation of basin edge generated waves and natural resonances within the los angeles basin the effect of the mountains reduces the peak amplitude of ground velocity for some regions in the basin by as much as 50 in the frequency band up to 0 5 hz these results suggest that depending on the relative location of faulting and the nearby large scale topography the topography can shield some areas from ground shaking

wave friction factors in nearshore sands abstract wave friction factors are estimated from vertical profiles of near bed turbulence and horizontal velocity spanning the wave bottom boundary layer measured values are partitioned by bed state which ranged from irregular ripples to flat bed and are examined as a function of two traditionally selected parameters physical bed roughness and outer flow reynolds number the measurements are from two field experiments in very different nearshore environments a relatively protected unbarred pocket beach and a linear barred beach exposed to the open shelf duck measured wave friction factors are remarkably similar for the two beaches and are highest for low energy rippled beds and lowest for the high energy flat bed conditions the reduction in the friction factor for high energy conditions corresponds to a decrease in the physical roughness of the bed as wave energy increases as a function of relative roughness measured friction factors are generally consistent with previous laboratory results and

principal bed states during sandyduck97 occurrence spectral anisotropy and the bed state storm cycle 1 results are presented from 70 days of nearly continuous in situ acoustic imagery of the nearshore sandy seabed in 3 m mean water depth at two locations separated by 40 m cross shore distance the bottom sediments were 150 m median diameter sand with nearly identical size distributions at the two locations five principal bed states were observed irregular ripples cross ripples linear transition ripples lunate megaripples and flat bed the linear transition and flat bed states were the most frequent together accounting for 68 of the total time bed state occurrence was a strong function of incident wave energy each bed state occurring within a relatively narrow range of sea and swell energies during the 12 major storm events spanned by the record the bed response was characterized by a repeatable bed state storm cycle involving four of the five principal states lunate megaripples did not appear repeatedly and thus may be a special case with no obvious dependence of bed state occurrence on prior bed state or on third moment measures of wave nonlinearity radial spectra from the rotary acoustic images indicate pronounced differences in the anisotropy of spatial scales for the different bed states and exhibit onshore offshore differences which are likely related to ripple migration

movable bed roughness in unsteady oscillatory flow a model to predict the roughness in unsteady oscillatory flows over movable noncohesive beds is presented the roughness over movable beds is shown to be a function of the boundary shear stress rather than a fixed geometrical scale as is the case for fully rough turbulent boundary shear flows over immobile beds the model partitions the roughness into two distinct contributions these two contributions are due to the form drag around individual bed forms and to the near bed sediment transport the form drag over the bed forms is treated explicitly as a function of the boundary geometry and shear stress the ripples are predicted as a function of the local skin friction and a semiempirical expression is derived using standard law of the wall arguments which gives the ripple or form roughness as a function of the boundary geometry the ripple roughness is found to be proportional to the product of the ripple steepness and height favorable comparison of the form drag model with the results of bagnold s 1946 fixed ripple study is found the value of z0 associated with intense sediment transport in oscillatory flow over a flat bed is determined from carstens et al s 1969 experiments this value is found to be 7 or 8 grain diameters an expression is derived for the roughness associated with the maximum thickness of a near bottom sediment transporting layer consistent with owen s 1964 roughness hypothesis for saltation of uniform grains in air at large values of the boundary shear stress relative to the critical value for initial sediment motion the derived expression is similar to the results of smith and mclean s 1977 unidirectional flow approach modified for oscillatory flow the total roughness model is found to compare favorably with carstens et al s 1969 data in contrast to smith and mclean s 1977 steady flow findings the results here show that when ripples are present they account for a significant portion of the boundary roughness