ALBERT

Description: Abstract Organic‐rich mudstones have long been of interest as conventional and unconventional source rocks and are an important organic carbon sink. Yet the processes that deposited organic‐rich muds in epicontinental seaways are poorly understood, partly because few modern analogues exist. This study investigates the processes that transported and deposited sediment and organic matter through part of the Bowland Shale Formation, from the Mississippian Rheic–Tethys seaway. Field to micron‐scale sedimentological analysis reveals a heterogeneous succession of carbonate‐rich, siliceous, and siliciclastic, argillaceous muds. Deposition of these facies at basinal and slope locations was moderated by progradation of the nearby Pendle delta system, fourth‐order eustatic sea‐level fluctuation and localized block‐and‐basin tectonism. Marine transgressions deposited bioclastic ‘marine band' (hemi)pelagic packages. These include abundant euhaline macrofaunal tests, and phosphatic concretions of organic matter and radiolarian tests interpreted as faecal pellets sourced from a productive water column. Lens‐rich (lenticular) mudstones, hybrid, debrite and turbidite beds successively overlie marine band packages and suggest reducing basin accommodation promoted sediment deposition via laminar and hybrid flows sourced from the basin margins. Mud lenses in lenticular mudstones lack organic linings and bioclasts and are equant in early‐cemented lenses and in plan‐view and are largest and most abundant in mudstones overlying marine band packages. Thus, lenses likely represent partially consolidated mud clasts that were scoured and transported in bedload from the shelf or proximal slope, as a ‘shelf to basin' conveyor, during periods of reduced basin accommodation. Candidate in situ microbial mats in strongly lenticular mudstones, and as rip‐up fragments in the down‐dip hybrid beds, suggest that these were potentially key biostabilizers of mud. Deltaic mud export was fast, despite the intrabasinal complexity, likely an order of magnitude higher than similar successions deposited in North America. Epicontinental basins remotely linked to delta systems were therefore capable of rapidly accumulating both sediment and organic matter. This article is protected by copyright. All rights reserved.

Description: Abstract Deep sedimentary basins amplify long‐period shaking from seismic waves, increasing the seismic hazard for cities sited on such basins. We perform 3‐D simulations of point source earthquakes distributed around the Seattle and Tacoma basins in Washington State to examine the dependence of basin amplification on source azimuth, depth, and earthquake type. For periods between 1 and 10 s, the pattern of amplification is spatially heterogeneous and differs considerably with the source‐to‐site azimuth. For close‐in earthquakes, the greatest basin amplification occurs toward the farside of the basin and ground motions from crustal earthquakes experience greater amplification than those from more vertically incident, deeper intraplate earthquakes. Love and Rayleigh waves form similar spatial patterns for a given source location, although the magnitude of amplification varies. The source dependence of basin amplification is an important factor for seismic hazard assessment, in both the Seattle and Tacoma basins, and by extension for deep sedimentary basins worldwide.

Description: 〈span〉〈div〉ABSTRACT〈/div〉Characterizing earthquake ground motions through 3D simulations is becoming standard practice for seismic hazard assessment in urbanized regions. However, accurate ground‐motion predictions require shear‐wave velocity (VS) data at depths that capture the extent of the sedimentary column (usually greater than 30 m), which can be difficult to obtain. We acquired microtremor array data at 11 sites in the Seattle basin, Washington, and applied the wavenumber‐normalized spatial autocorrelation (SPAC) method (krSPAC) to obtain VS at depths as great as 2200 m. In a traditional SPAC approach, modeling high wavenumbers within the SPAC spectrum requires array symmetry. By contrast, in the krSPAC approach we transform observed coherency versus frequency spectra to coherency versus kr (in which k and r are wavenumber and station separation, respectively) prior to VS modeling. Through this transformation, the requirement for array symmetry is eased. We deployed seven‐sensor nested irregular triangular arrays, with nominal interstation spacings that varied from about 300 to 2000 m. Comparison of VS derived from krSPAC to a previous interpretation from ambient‐noise tomography studies suggests a broadly comparable VS structure in the 250–1000 m depth range with improved resolution at shallower depth. At each site, we interpret a high‐velocity Quaternary boundary in which VS increases above 900  m/s. Using this boundary as the reference horizon, we calculate ground‐motion amplification of a factor of up to 2 from the overlying Quaternary sediments between 0.3 and 7 Hz, assuming vertically propagating 〈span〉S〈/span〉 waves.〈/span〉

Description: Abstract Empirical equations for wave breaking and wave setup are compared with archived shoreline wave setup measurements to investigate the contribution of wind‐waves to extreme Mean Total Water Levels (MTWL, the mean height of the shoreline), for natural beaches exposed to open ocean wind‐waves. A broad range of formulations are compared through linear regression and quantile regression analysis of the highest measured values. Shoreline wave setup equations are selected based on the availability of local beach slope data and the ability of the quantile regression to show a good representation of the highest measured levels. Wave parameters from an existing spectral wave hindcast are used as input to the selected equations and are combined with a storm‐tide time series to quantify the relative contribution of shoreline wave setup to the extreme MTWL climate along Australian beaches. A multi‐pass analysis is provided to understand the ability to capture the shoreline wave setup estimates with and without considering beach slope. The national scale analysis which does not include beach slope indicates there are multiple contributing factors to MTWL. Examples are provided at two locations of differing local beach slope to show the importance of including local beach slope in determining the contribution of waves to MTWL. A tool is in development for further investigation of wave setup for Australian beaches.

Description: The microtopography of two sandstone blocks with and without colonization of biofilms were measured with a traversing micro‐erosoin meter (TMEM) under different simulated environmental conditions. Two‐hourly microtopographic fluctuations of supratidal sandstone were mainly induced by the colonized biofilms and influenced by environmental factors. By increasing the magnitude and number of cycles of expansion and contraction, lithobiontic biofilms were proposed to play an erosive role in rock decay at hourly scale. Abstract In this study laboratory experiments were used to explore the role of biofilms, formed by lithobiontic microorganism communities, in causing hourly surface changes of supratidal sandstone and the potential linkage to long‐term rock decay. To isolate the influence of individual environmental factors (temperature and humidity) on rock surface changes (expansion and contraction), a colonized (biofilm‐covered) and a non‐colonized sandstone block (biofilm‐free) underwent the same univariate microclimatic simulations closely controlled by an environmental chamber. Simulations were run under three different light conditions, with a natural light lamp on, on and off at 20‐min intervals and off, to investigate the impact of light on rock surface dynamics. Measured with a traversing micro‐erosion meter (TMEM), two‐hourly microtopographic fluctuations of these two sandstone blocks were compared in the same environment. Induced by microclimatic variations, surface movements of significantly higher magnitude (12–120% under varying tempeature and 121–154% under varying humidity) and different change patterns were observed on the colonized block, indicating the primary role of biofilm in driving microtopographic fluctuations of supratidal sandstone. However, thermally driven changes of similar magnitude and pattern were observed on both surfaces, suggesting other mechanisms also operating on the non‐colonized rock surface in this process. Due to the sensitivity of biofilm microorganism communities to light, the magnitude and pattern of surface changes was impacted by light condition. Because biofilms increased the magnitude and number of cycles of expansion and contraction of the experimental rock surface, we propose that lithobiontic biofilms facilitate the detachment of grains and granular disintegration on the rock surface, consequently contributing to rock decay and accelerating the rate of breakdown of supratidal rock. This short‐term episode therefore needs to be superimposed on longer term studies to fully understand the role of biofilms in rock surface change. © 2019 John Wiley & Sons, Ltd.

Description: Ensemble forecasts are routinely used as a basis for probabilistic predictions. The skill of probabilistic predictions derived from ensemble forecasts depends on the number of ensemble members. We derive a new verification score, called the ensemble‐adjusted Ignorance Score, which can correct for the effect of limited ensemble size and therefore allows for a more robust comparison of forecasts based on different ensemble sizes. The unadjusted Ignorance Score (solid line) depends on the ensemble size m, assigning higher (worse) scores to smaller ensembles drawn from the same forecast distribution. The ensemble‐adjusted Ignorance Score (dashed line) proposed here does not depend on ensemble size and thus allows for a fair comparison of equivalent ensembles of different sizes. This study considers the application of the Ignorance Score (IS, also known as the Logarithmic Score) for ensemble verification. In particular, we consider the case where an ensemble forecast is transformed to a normal forecast distribution, and this distribution is evaluated by the IS. It is shown that the IS systematically depends on the ensemble size, such that larger ensembles yield better expected scores. An ensemble‐adjusted IS is proposed, which extrapolates the score of an m‐member ensemble to the score that the ensemble would achieve if it had fewer or more than m members. Using the ensemble adjustment, a fair version of the IS is derived, which is optimized if ensembles are statistically consistent with the observations. The benefit of the ensemble adjustment is illustrated by comparing ISs of ensembles of different sizes in a seasonal climate forecasting context and a medium‐range weather forecasting context. An ensemble‐adjusted score can be used for a fair comparison between ensembles of different sizes, and to accurately estimate the expected score of a large operational ensemble by running a much smaller hindcast ensemble.

Description: 〈span〉〈div〉ABSTRACT〈/div〉Characterizing earthquake ground motions through 3D simulations is becoming standard practice for seismic hazard assessment in urbanized regions. However, accurate ground‐motion predictions require shear‐wave velocity (VS) data at depths that capture the extent of the sedimentary column (usually greater than 30 m), which can be difficult to obtain. We acquired microtremor array data at 11 sites in the Seattle basin, Washington, and applied the wavenumber‐normalized spatial autocorrelation (SPAC) method (krSPAC) to obtain VS at depths as great as 2200 m. In a traditional SPAC approach, modeling high wavenumbers within the SPAC spectrum requires array symmetry. By contrast, in the krSPAC approach we transform observed coherency versus frequency spectra to coherency versus kr (in which k and r are wavenumber and station separation, respectively) prior to VS modeling. Through this transformation, the requirement for array symmetry is eased. We deployed seven‐sensor nested irregular triangular arrays, with nominal interstation spacings that varied from about 300 to 2000 m. Comparison of VS derived from krSPAC to a previous interpretation from ambient‐noise tomography studies suggests a broadly comparable VS structure in the 250–1000 m depth range with improved resolution at shallower depth. At each site, we interpret a high‐velocity Quaternary boundary in which VS increases above 900  m/s. Using this boundary as the reference horizon, we calculate ground‐motion amplification of a factor of up to 2 from the overlying Quaternary sediments between 0.3 and 7 Hz, assuming vertically propagating 〈span〉S〈/span〉 waves.〈/span〉