ALBERT

Description: Seismic facies analysis can generate a map to describe the spatial distribution characteristics of reservoirs, and therefore plays a critical role in seismic interpretation. To analyse the characteristics of the horizon of interest, it is usually necessary to extract seismic waveforms along the target horizon using a selected time window. The inaccuracy of horizon interpretation often produces some inconsistent phases and leads to inaccurate classification. Therefore, the developed adaptive phase K-means algorithm proposed a sliding time window to extract seismic waveforms. However, setting the maximum offset of the sliding window is difficult in a real data application. A value that is too large may cause the cross-layer problem, whereas a value that is too small reduces the flexibility of the algorithm. To address this disadvantage, this paper proposes a robust K-means (R-K-means) algorithm with a Gaussian-weighted sliding window for seismic waveform classification. The used weights punish those windows distant from the interpretation horizon in the objective function, consequently producing a smaller range of horizon adjustments even when using relatively large maximum offsets and benefitting the generation of stable and reliable seismic facies maps. The application of real seismic data from the F3 block proves the effectiveness of the proposed algorithm.

Description: As the robustness for the wave equation-based inversion methods, wave equation migration velocity analysis (WEMVA) is stable for overcoming the multipathing problem and has become popular in recent years. As a rapidly developed method, differential semblance optimisation (DSO) is convenient to implement and can automatically detect the moveout existing in common image gathers (CIGs). However, by implementing in the image domain with the target of minimising moveouts and improving coherence of the CIGs, the DSO method often suffers from imaging artefacts caused by uneven illumination and irregular observation geometry, which may produce poor velocity updates with artefact contamination. To deal with this issue, in this paper, by introducing Wiener-like filters, we modify the conventional image matching-based objective function to a new one by introducing the quadratic Wasserstein metric technique. The new misfit function measures the distance of two distributions obtained by the convolutional filters and target functions. With the new misfit function, the adjoint sources and the corresponding gradients are improved. We apply the new method to two numerical examples and one field dataset. The corresponding results indicate that the new method is robust to compensate low frequency components of velocity models.

Description: Atlantic salmon (Salmo salar) populations throughout the North Atlantic have declined in recent decades largely due to reduced marine survival, yet our understanding of marine distribution patterns and migratory routes remains limited. Here, we assigned archived individual samples (n = 3891) collected over a half century (1968–2018) throughout the North Atlantic to region of origin using range-wide genetic assignment. In the Northwest Atlantic, the distribution of assignments reinforced the importance of the Labrador Sea as an aggregation area, with 73% of all reporting groups detected. Moreover, individuals from six European reporting groups were identified in the Northwest Atlantic, and detections decreased with decreasing latitude spanning an area from Greenland to southern Newfoundland. In the Northeast Atlantic, six North American reporting groups were detected in samples from around the Faroe Islands. Based on the distribution of samples, estimates of trans-Atlantic migration distance averaged 3861 and 2889 km for North American and European salmon respectively. Our analysis highlights the widespread importance of the Labrador Sea and Faroe Islands to the species marine distribution patterns, and the prevalence of long-distance trans-Atlantic migration. Ultimately, the results suggest that environmental conditions experienced by many Atlantic salmon populations span much of the North Atlantic Ocean.

Description: Heterotrophic protists are essential components of the marine ecosystem, yet they are often excluded from monitoring programmes. With limited resources, monitoring strategies need to be optimised considering both scientific knowledge and available resources. In doing so, it is crucial to understand how sampling frequency affects the value of the data. We analysed 11 years of weekly heterotrophic protist time-series data from Station L4 in the Western English Channel to explore how different sampling intervals impact data quality. In the L4 dataset, comprising 55 protist taxa, the reduction of sampling frequency from weekly to four times a year at specific seasons decreased the number of taxa encountered by 38% for ciliates and 29% for heterotrophic dinoflagellates while the mean annual biomass or its mean variation were not affected. Furthermore, when samples were taken only four times a year, biomass peaks of the ten most important taxa were often missed. The primary motivator for this study was furthering the development of the heterotrophic protist monitoring in temperate and subarctic marine areas, e.g. the Baltic Sea. Based on our findings, we give recommendations on sampling frequency to optimise the value of heterotrophic protist monitoring.

Description: The Lafaye orbicular body was emplaced in the Villatange tonalite-granodiorite unit of the Guéret magmatic complex (Massif Central, France). It consists of plagioclasic orbicules (4–35 cm diameter) embedded in homogeneous cordierite granodiorite. Orbicule cores consist mostly of residual metasedimentary xenoliths or autolithic plagioclasic cumulates. Rims (0.7–8 cm thickness) are single- or multi-layered; layers, mostly comb-textured, comprise alternating sheets dominated by cordierite (XFe = 0.32–0.37) or plagioclase (mostly An25–30). Additional mineral phases are minor biotite (XFe = 0.52; AlVI = 0.58–0.92 atoms per formula unit) and interstitial quartz. Plagioclase and cordierite morphologies (needle-like, skeletal, branching or fan-shaped) indicate growth under high initial supersaturation. However, the final polyhedral shapes and primary zoning of many individual plagioclase crystals, as well as evidence of partial recrystallization, imply significant textural maturation. Whole-rock major and trace element data (A/CNK = 1.12–1.46) and Sr and Nd isotopic compositions (εNd(355 Ma)  = −8.6 to −7.4; 87Sr/86Sr(355 Ma) = 0.7110–0.7147) suggest that the parental magma of the orbicules resulted from bulk assimilation of aluminous metasediments by a Villatange-type granodioritic magma. Heterogeneous nucleation and growth of plagioclase and cordierite around xenoliths/autoliths are interpreted in terms of (1) adiabatic decompression of magma pulses ascending in dykes leading to superheating and resorption of early solids, and (2) volatile exsolution, inducing undercooling, supersaturation, and rim crystallization. The variability of layers (number, thickness, mineral distribution, and texture) is considered to result from oscillatory crystallization combined with variable plagioclase growth rates linked to changes in the degree of supersaturation as a function of the extent of melt degassing, itself linked to magma transfer dynamics.

Description: La Réunion Island includes two major volcanic systems. About 0·5 Myr ago, Piton des Neiges volcano declined, while Piton de la Fournaise volcano grew on its flank. Since then the Piton de la Fournaise shield volcano has produced homogeneous lavas with chemical compositions transitional between alkali and tholeiitic basalts. In April 2007, the volcano emitted a very small volume of trachytic pumice during its largest historical eruption. We conducted a comprehensive petrological and geochemical study of the pumice to understand the occurrence of such silicic melt in the feeding system of this highly active basaltic volcano. Isotopes of Sr, Nd, Pb and O, together with trace elements, indicate that the trachyte is genetically related to the La Réunion mantle plume and derives from crystallization of a typical basalt. The trachyte chemistry records a long and complex history of differentiation and outgassing. The extensive depletion of moderately volatile elements (F, Cl, B, Cs, Cu, Li) and less volatile uranium is consistent with exsolution of dense fluids at depths of several kilometres. Lithium isotopes point to closed-system degassing during the very late stages of crystallization. U-series isotopes and radiogenic 208Pb*/206Pb* constrain the age of U loss to between 0·4 and 2·1 Ma. This age is as old as or older than the Piton de la Fournaise shield edifice. The 2007 trachyte could thus be a liquid remnant of an extinct volcano, such as Piton des Neiges or Les Alizés (Piton de la Fournaise proto-volcano). It could also result from partial melting of an old syenite intrusion or remobilization of interstitial melts not fully solidified. Thermal modelling indicates that the sustained heat flux from hot basaltic magmas rising from the mantle can maintain temperatures above 800 °C in the central feeding system, and prevent total solidification of magmas trapped in this hot core.

Description: Fisheries management is usually supported by technical and financial measurements (i.e. logbooks and market data), which are helpful for ecological or economic assessments. Yet this information is not able to address social heterogeneity and fisher motivations, which are key to understanding fisher behaviour. This case study of the demersal segment in the Netherlands shows that combining quantitative analysis of logbooks with qualitative data collected by engaging with fishers can capture both fishing activity and its motivations, generating a more social understanding of fisher behaviour. A métier analysis of logbook data describes five dominant fishing practices among the selected segment. Twenty-five in-depth interviews with fishers along with focus groups including other experts identify three social factors that influence fisher behaviour in the Dutch demersal fleet: business structure, working rhythm, and polyvalence. The results show that motivations for fisher behaviour are more complex than complying with regulations or seeking profit: social factors also influence fishing activity. Furthermore, these social factors have real implications for the impacts of management measures on both the fishing communities and the environment, especially in times of change. These results are useful for management strategy development or evaluation because they are feasibly observable through existing data collection protocols.

Description: Harbour seals are surveyed aerially when they haul-out to moult in August. The proportion of the population hauled out throughout the year is related to temporal, environmental, and meteorological variables. Thus, monitoring is conducted under predefined ranges of conditions. Effects of variation within these ranges are rarely reviewed. We used linear models to assess effects of time, date and weather on the difference between counts predicted by a population growth model and observed counts, based on a 30-year time-series. Our top-ranked model explained 34.4% of the variance. Survey date and its interaction with survey year were the most important variables, with higher counts earlier in August, particularly early in the time series, where surveys may not have been timed optimally to capture the peak in the moult. Cloud cover, wind speed, temperature, and interactions between these were of lesser importance; there were fewer seals on land during cloudy, windy days and on clear, warm days. These effects of weather are likely related to temperature regulation. Power analyses suggested that correction for survey conditions would allow detection of a one percentage point annual change in population growth rate with 80% power 4 years sooner than without taking survey conditions into account.

Description: Sargassum forests play an important role in coastal waters as habitats for marine organisms, including commercial species. However, human activities have negatively affected their distribution causing a worldwide decline of Sargassum forests. Mapping and monitoring the distribution and biomass of these habitats using acoustic remote sensing techniques is key for their conservation. Nonetheless, most researches based on acoustic remote sensing methods focus on estimations of macrophyte area and its canopy height, and less researches reporting 3D visualization of these habitats. This study demonstrates the use of high-resolution multibeam echosounder (MBES) bathymetric data to visualize the 3D structure of Sargassum forests. Comparing acoustic data and underwater camera photos collected in field surveys, we identified Sargassum individuals as vertical clusters of contiguous sounding points with a base close to the sea bottom in the sounding data of the MBES. Using this criterion, we could distinguish Sargassum echoes, visualize the 3D structure of Sargassum forests and estimate the number of Sargassum individuals in the survey area. Using the relation between thallus length and dry weight of sampled Sargassum plants, standing stock and biomass could be estimated assuming the thallus length was the height of Sargassum plants identified with the MBES.

Description: This paper reviews application of the Daily Egg Production Method (DEPM) to sardine (Sardinops sagax) off southern Australia between 1995 and 2019. Coefficients of variation (CVs) of estimates of spawning biomass (SB) were reduced from 23–59% to 8–12% by: (i) estimating mean daily egg production (P0), spawning fraction (S), and sex ratio (R) from all historical data rather than annually; and (ii) combining batch fecundity (F) and female weight (W) into a single parameter, relative fecundity (F′ = F^/W). Total daily egg production was estimated most precisely from annual estimates of spawning area (A) and estimates of P0 obtained from historical data. Both S and R were estimated most precisely from historical data. Estimating W and F from historical data did not increase precision. F′ had lower CVs than both W and F, and was stable across years and a wide range of W. Findings demonstrate that A can be converted into a precise estimates of SB using estimates of P0, S, R, and F′ obtained from historical data. However, the possibility that DEPM parameters may change in the future cannot be discounted. Future monitoring should include annual estimation of P0 and periodic (e.g. 3–5 years) re-estimation of adult parameters.

Description: In his 1953 paper, Harald Sverdrup argued that the development of a spring bloom in the ocean depends on the juxtaposition of two depth horizons: the mixed-layer depth and the critical depth. Mixed-layer depth shallower than the critical depth favours phytoplankton growth in the layer and vice versa. However, mathematically, Sverdrup left the problem unsolved in the form of a transcendental equation. In spite of the high number of citations that this paper has garnered, the solution to this equation has not been found, until now. In this work, we present an analytical solution for the critical depth, as originally defined by Sverdrup. The paper opens with the definition of the critical depth and the description of the Lambert W function. The analytical solution for critical depth follows. Sverdrup’s original model is extended to include the effect of light attenuation by phytoplankton and the analytical solution for steady-state biomass in the mixed layer is derived. The expression for mixed-layer production at steady state is also presented. Two novel variants of the critical depth are defined: the optically uncoupled critical depth and the optically coupled critical depth. It is demonstrated that at steady state the optically coupled critical depth equals the mixed-layer depth and that the irradiance at the base of the mixed layer equals the irradiance at the optically uncoupled critical depth. Competitive exclusion is demonstrated to hold and the optically uncoupled critical depth is linked to the critical light intensity in multi-species competition. Finally, a conservation principle for the critical depth is found.

Description: Fishery bycatch poses a serious threat to seabird populations globally. Traditional haul-only post-capture observations are inadequate and inefficient to document seabird bycatch due to the substantial bycatch loss known to occur. Pre-capture observations offer an alternative by documenting seabird interactions leading up to captures. Based on the long-term large-scale dedicated field observations, this study revealed significant risk factors for the pre-capture stages of the seabird bycatch process in pelagic longline fisheries using Bayesian methods. Rough sea conditions were found to correlate with more seabirds following fishing vessels. Species identity, density effect, inter-species effect, and sea condition were found to significantly affect how frequently seabirds aggregated around a fishing vessel engage in bait-taking interactions. Intra-species competition was found to be the dominant type of density effect. Moreover, a web of inter-species interactions was identified to facilitate the bait-taking of superior competitors at the expense of inferior ones. The findings of this study are relevant to fishery managers in updating current data collection protocols to alleviate data issues caused by bycatch loss, to conservation biologists in quantifying bycatch risks for susceptible seabird populations, and in aiding the design and evaluation of bycatch mitigation measures.

Description: Fishing and translocation of marine species for use in aquaculture is widespread. Corkwing, goldsinny, and ballan wrasse (Symphodus melops, Ctenolabrus rupestris, and Labrus bergylta) are fished on the Swedish west coast for use as cleaner-fish in Norwegian salmon farms. Here, we aim to provide knowledge and recommendations to support ecosystem-based management for wrasse fisheries in Sweden. We compared fished and non-fished areas to test if current fishery levels have led to stock depletion. To gain insight on the role of wrasse in the algal belt trophic chain, we analysed the gut contents of goldsinny and corkwing using metabarcoding. Finally, we analysed the trophic interactions of wrasse and potential prey in a mesocosm study. We could not detect any signs of stock depletion or altered size structure in fished areas compared to the protected control area. Gut analyses confirmed both goldsinny and corkwing as non-specialized, omnivorous opportunists and revealed, with 189 prey taxa detected, a broader spectrum of prey than previously known. Common prey items included mesoherbivores such as small gastropods and crustaceans, but also insects and algae. We conclude that there are no visible signs of stock depletion at the current removal level of wrasses by the fishery. However, this emerging fishery should be closely monitored for potential cascading effects on the algal belt ecosystem, and our study could provide a baseline for future monitoring.

Description: Target strength model inputs including morphometry, material properties, lipid composition, and in situ orientations were measured for sub-Arctic krill (Euphausia pacifica, Thysanoessa spinifera, T. inermis, and T. raschii) in the eastern Bering Sea (EBS, 2016) and Gulf of Alaska (GOA, 2017). Inter-species and -regional animal lengths were significantly different (F1,680 = 114.10, p 

Description: Atlantic cod (Gadus morhua) is one of the most commercially important fish species in the North Atlantic. Environmental factors, such as water temperatures, influence growth of individuals over time, thus forming population-specific growth patterns across climatic regions. Here we develop an integrative approach to investigate the role of temperature in shaping geographic differences of cod growth in the Celtic Sea, North Sea, Iceland, and Barents Sea. We combine a physiology-based growth model and 50-years observational temperature data of 0.5 × 0.5° spatial resolution to simulate continuous growth of cod. The model generated weight-at-age data for the period 1959–2007 which we compared to observational data from fishery-independent scientific surveys. In the Celtic and the northern North Sea, simulated growth matches well observational data. We also show that relatively warm temperatures in the Celtic Sea facilitate maximum growth rates; future warming is likely to have a negative impact on growth of these cod stocks. Growth simulations in Icelandic waters and the Barents Sea are less consistent with local observational data. More complex growth patterns in these regions are probably shaped by ontogenetic shifts in temperature regimes, feeding conditions and physiological adaptations. These findings should stimulate further research on critical processes to be considered in population-specific projections of growth of cod and productivity.

Description: In designing and performing surveys of animal abundance, monitoring programs often struggle to determine the sampling intensity and design required to achieve their objectives, and this problem greatly increases in complexity for multispecies surveys with inherent trade-offs among species. To address these issues, we conducted a multispecies stratified random survey design optimization using a spatiotemporal operating model and a genetic algorithm that optimizes both the stratification (defined by depth and longitude) and the minimum optimal allocation of samples across strata subject to prespecified precision limits. Surveys were then simulated under those optimized designs and performance was evaluated by calculating the precision and accuracy of a resulting design-based abundance index. We applied this framework to a multispecies fishery-independent bottom trawl survey in the Gulf of Alaska, USA. Incorporating only spatial variation in the optimization failed to produce population estimates within the prespecified precision constraints, whereas including additional spatiotemporal variation ensured that estimates were both unbiased and within prespecified precision constraints. In general, results were not sensitive to the number of strata in the optimized solutions. This optimization approach provides an objective quantitative framework for designing new, or improving existing, survey designs for many different ecosystems.

Description: Changes in animal movement and behaviour at fine scales (tens of metres) in immediate proximity to tidal stream turbine structures are largely unknown and have implications for risks of animal collision with turbine blades. This study used upward-facing multibeam echosounder data to detect and track animal movement comprising fish, diving seabirds, and marine mammals. Measurements over spring-neap tidal cycles at a turbine structure (no blades present) are compared to a neighbouring reference area with no structure and comparable conditions, with measurements consecutive in time to maximize comparability. The majority of tracked animals (93.4% around turbine structure and 99.1% without turbine structure) were observed swimming against the flow, with 87.5% and 97.8%, respectively, making ground and showing capability of manoeuvring in tidal stream flow speeds. Track tortuosity increased around the turbine structure compared to the reference site, particularly in the wake and at low flow speeds, indicating animal station-holding or milling behaviour. These data also evidence the benefits of multibeam echosounders to measure animal movement through larger measurement volumes rather than relying on single-beam echosounders to measure animal presence alone, including to avoid large biases overestimating the size of schools swimming against the flow measured by time-in-beam.

Description: The Antarctic Peninsula is one of the most rapidly warming regions on earth, and it is likely that the abundance and distribution of marine predators will change as a result.Procellariiform seabirds are highly mobile predators, which target specific habitat characteristics associated with underlying distributions of prey and areas of increased prey availability. We use ship surveys and hurdle models, to estimate the summer distribution and relative density of 11 seabird species within the northern Antarctic Peninsula marine ecosystem. Models differed among species; however, sea surface temperature and depth were frequently associated with seabird occurrence and had the greatest explanatory power across many species. Null models based on observation data were better at predicting seabird density than models that included environmental covariates. This suggests that the main driver of distribution patterns is the broad-scale habitat features, and fine-scale aggregations within these ranges are harder to predict. Our seabird distribution models reflect known habitat associations, species hotspots, and community organization relative to oceanic and coastal marine processes. Application of species distribution models will benefit the assessments of critical habitat and potential responses to climate change and anthropogenic disturbance, which will provide insight into how species may change in polar ecosystems.

Description: Many data-limited fish stocks worldwide require management advice. Simple empirical management procedures have been used to manage data-limited fisheries but do not necessarily ensure compliance with maximum sustainable yield objectives and precautionary principles. Genetic algorithms are efficient optimization procedures for which the objectives are formalized as a fitness function. This optimization can be included when testing management procedures in a management strategy evaluation. This study explored the application of a genetic algorithm to an empirical catch rule and found that this approach could substantially improve the performance of the catch rule. The optimized parameterization and the magnitude of the improvement were dependent on the specific stock, stock status, and definition of the fitness function. The genetic algorithm proved to be an efficient and automated method for tuning the catch rule and removed the need for manual intervention during the optimization process. Therefore, we conclude that the approach could also be applied to other management procedures, case-specific tuning, and even data-rich stocks. Finally, we recommend the phasing out of the current generic ICES “2 over 3” advice rule in favour of case-specific catch rules of the form tested here, although we caution that neither works well for fast-growing stocks.

Description: Ocean grabbing occurs when traditional users, such as small-scale fishers, are pushed aside by new development activities. This grabbing must be prevented to avoid sea uses that maintain or increase social inequity. In this paper, we show that in tropical Atlantic countries, such as Brazil and Senegal, examples of ocean grabbing already occur. In this context, we analyse if Maritime Spatial Planning (MSP) may be an opportunity to limit ocean grabbing or, to the contrary, poses a risk to increase it. MSP calls for an ecosystem approach that requires integrated coastal and marine management and involves stakeholders in developing a shared vision of the future, where society and environment are preserved. However, recent studies have shown that MSP is a process to be used cautiously to ensure equitable decisions. Meanwhile, the concept is spreading worldwide including in tropical Atlantic countries. We highlight that context matters and the specificities of the tropical Atlantic must be taken into account when deploying MSP processes. In the tropical Atlantic context, there is increased imbalances of stakeholder power, traps from decision support tools, and a need for adaptive management. These specific features must be addressed when deploying MSP in a way to avoid ocean grabbing.

Description: The influence of oceanographic variables on assemblages of meso- and bathypelagic fish was investigated along a Northeastern Atlantic Ocean transect (Cape Verde to the Bay of Biscay) during May 2019. Fish were collected using a mrozooplankton trawl during daylight hours at ten stations. Along the transect, 17 hydrographic stations were also performed with a CTD (Conductivity, Temperature, and Depth). A total of 130 fish taxa were identified. The dominant family was Gonostomatidae, with four species (Cyclothone braueri, Cyclothone microdon, Cyclothone pseudopallida, and Cyclothone pallida) being responsible of more than 78% of the total density. The most frequent species that appeared to be ubiquitous were C. braueri and C. pseudopallida, while Myctophidae was the most diverse family. Multivariate analyses revealed two clusters related with the latitudinal gradient. The fish community in the southern stations (25–37°N) was more diverse than in the northern stations (42–48°N). Temperature from 300 to 700 m depth explained 65% of variation in terms of density and 58% in terms of biomass, both statistically significant. The investigated variation in the deep-pelagic ecosystems on a large spatial scale gives essential information to ecosystem management approhes and marine spatial planning.

Description: In the European Union (EU), the ecosystem approach to fisheries is implemented through several directives and polices, which are overarched by the Marine Strategy Framework Directive (MSFD). The MSFD requires the assessment of the environmental status of exploited fish and shellfish stocks, among others, to advise the EU Common Fisheries Policy (CFP) on sustainable catch options. The stock assessments for the CFP and the MSFD are supposed to be congruent, yet they differ in several substantial subjects. While the annual CFP assessment is based on two indicators, fishing mortality (F) and spawning stock biomass (SSB), in its Descriptor 3 (D3) the MSFD requires the assessment of three criteria (F, SSB, and age or size structure) within a six year period. Assessing exploited fish and shellfish stocks under the MSFD by using existing CFP assessments has therefore been a half-completed task, which had to be fulfilled by the member states of the EU. This paper suggests six easy steps, referred to as “quick wins” (QW), which are based on existing information from CFP stock assessments. The implementation of these six QW would allow for assessments of exploited fish and shellfish stocks that are compliant to D3. These QW are to (i) assess length/age structure within a stock, (ii) analyse and assess selectivity by fisheries, (iii) use all available information to assess a stock, (iv) use response indicators to assess environmental targets, (v) provide integrated stock-specific advice, and (vi) provide assessments with a mid-term perspective over a-period of six years. International Council for the Exploration of the Sea (ICES) combines the infrastructure and expertise to produce stock-specific assessment products tailored to the requirements of MSFD D3. Thereby, ICES could provide a valuable service to EU member states in the north-east Atlantic region by providing scientifically validated, quality-assured, and MSFD-compliant single-stock assessment products. This would be a big advancement towards implementing the ecosystem approach to fisheries management within Europe.

Description: Managers, stakeholders, and scientists recognize the need for collaborative, transparent, integrated approaches to complex resource management issues, and frameworks to address these complex issues are developing. Through the course of 2019, the Mid-Atlantic Fishery Management Council developed a conceptual model of ecosystem linkages and risks for summer flounder, a species of recreational and commercial fisheries importance. The proximal aim of the model was to develop a list of integrated management questions that could be refined and addressed through a future quantitative management strategy evaluation. As such, this conceptual model served as a scoping tool. However, the true value of the conceptual model lays elsewhere: familiarizing resource managers historically focused on single-species management with the potential utility of an ecosystem approach to management. This paper details the goals and development of the conceptual model and situates this process in the broader context of best practices for collaborative open science and scientific reproducibility. Further, it highlights a successful path by which the shift towards ecosystem-based management can be actuated.

Description: Understanding the competitive interactions of ecological similar species is essential to determine their role and niche in the ecosystem. Using both conventional dietary methods and stable isotope analysis, we examined the feeding ecology, trophic position and possible niche partitioning of Northeast Atlantic mackerel (Scomber scombrus), Icelandic summer spawning (ISS) herring (Clupea harengus), and Norwegian spring spawning (NSS) herring from Icelandic waters during the spring and summer 2012 and 2014. The stomach analysis showed differences in prey preferences among the species during summer, where mackerel diets were almost exclusively calanoid copepods, while herring ate larger zooplankton, i.e. euphausiids and amphipods. Analysis of isotopic diet contribution of mackerel and herring using Bayesian mixing models, representing mainly the spring consumption, revealed that euphausiids were the main dietary contributor of both mackerel and herring. Fish prey were also an important dietary source for ISS herring, and amphipods were important to the diet of NSS herring. In addition, mackerel encompassed a broad isotopic niche, whereas the two herring stocks had narrower and separated niches from each other. The results from this analysis reveal new and holistic information into the diet of these species, which have various ecological implications.

Description: Theoretical size at the onset of maturity (TSOM) for female Norway lobster was estimated by a new methodology based on the probability distributions of mature individuals built on physiological maturity measures. Onset of maturity using TSOM varied from 18.4- to 33.7-mm carapace length for the Irish functional management units (FUs). These estimates showed a significant negative linear relationship (R2 = 0.60) with population density and a significant positive linear relationship with average size in females (R2 = 0.84). The size class at which 50% of the females are sexually mature (L50) was linked to the new TSOM metric by a significant positive linear relationship (R2 = 0.40). This set of linear relationships ultimately allowed TSOM and L50 to be estimated without a requirement for maturity stages to be distinguished. As well as contributing to the stock assessment and management of Nephrops (e.g. in data-limited FUs) and its potential for the calibration of more routinely used estimates, TSOM might be applied in new species and meta-analyses where size of maturity data are scarce. This new metric also better-defines the maturity process since, taken together, TSOM, L50, and smallest berried female represent sequential maturity events: (i) onset of maturity, (ii) 50% mature (from gonad staging), and (iii) berried females.

Description: Although gelatinous zooplankton are an important component of marine ecosystems, gelatinous mesozooplankton that are

Description: The carapace length (CL) at which American lobster (Homarus americanus) females reach maturity can be used to evaluate egg production, growth patterns, and the overall health of lobster stocks. The female maturity datasets used to represent Gulf of Maine (GOM) lobsters in the 2015 Atlantic States Marine Fisheries Commission American Lobster Stock Assessment were collected in the 1990s by the Maine Department of Marine Resources at two coastal sites. Many studies have demonstrated an inverse relationship between temperature and the size at maturity in female lobsters, and GOM waters have warmed significantly over this period. To update these GOM maturity datasets, we used ovarian staging to determine the maturity status of over 1200 females from fives sites over 3 years. Broad application of this methodology in tandem with key growth measurements on females 50–120 mm CL allowed us to characterize reproductive development and generate maturity ogives (proportion mature at a given CL). We observed a latitudinal gradient in the size at maturity across this coastal region of the GOM and quantified a decrease in this size over 25 years. These findings have implications for future stock assessment approaches and management measures implemented to sustain this valuable fishery.

Description: The carbonate chemistry in coastal waters is more variable compared with that of open oceans, both in magnitude and time scale of its fluctuations. However, knowledge of the responses of coastal phytoplankton to dynamic changes in pH/pCO2 has been scarcely documented. Hence, we investigated the physiological performance of a coastal isolate of the coccolithophore Emiliania huxleyi (PML B92/11) under fluctuating and stable pCO2 regimes (steady ambient pCO2, 400 μatm; steady elevated pCO2, 1200 μatm; diurnally fluctuating elevated pCO2, 600–1800 μatm). Elevated pCO2 inhibited the calcification rate in both the steady and fluctuating regimes. However, higher specific growth rates and lower ratios of calcification to photosynthesis were detected in the cells grown under diurnally fluctuating elevated pCO2 conditions. The fluctuating pCO2 regime alleviated the negative effects of elevated pCO2 on effective photochemical quantum yield and relative photosynthetic electron transport rate compared with the steady elevated pCO2 treatment. Our results suggest that growth of E. huxleyi could benefit from diel fluctuations of pH/pCO2 under future-projected ocean acidification, but its calcification was reduced by the fluctuation and the increased concentration of CO2, reflecting a necessity to consider the influences of dynamic pH fluctuations on coastal carbon cycles associated with ocean global changes.

Description: Millions of people’s livelihoods rely on artisanal fisheries. However, in many regions fishers are increasingly facing ciguatera poisoning, a seafood-borne illness. The toxin, produced by benthic dinoflagellates, can spread through marine food webs and to humans by direct consumption. Ciguatera risk can play a major role in fisher’s activities but has never been considered in any marine spatial plans thus far. To fill this gap, we examined if integrating ciguatera in systematic conservation plans could affect these decisions. We developed through map-based interviews, a novel seven-step framework to collect and map local knowledge on ciguatera risk and fisheries activities with two innovations: (i) better mapping of fishing grounds by combining geomorphological habitat and fishing gear information, and (ii) integrating ciguatera risk directly into systematic spatial planning designs and scenarios conceived to maximize benthic habitat conservation while minimizing impacts to fishers. The approach is illustrated for Raivavae Island, in French Polynesia, Pacific Ocean. We found that integrating ciguatera significantly improved prioritization solutions with a 24–38% decrease of costs to fishers compared with scenarios based solely on fishery data. This framework was designed for scientists and managers to optimize the implementation of conservation plans and could be generalized to ciguatera-prone areas.

Description: Deep-sea marine fishes support important fisheries but estimates of their distributions are often incomplete as the data behind them may reflect fishing practices, access rights, or political boundaries, rather than actual geographic distributions. We use a simple suitable habitat model based on bottom depth, temperature, and salinity to estimate the potential distribution of Greenland halibut (Reinhardtius hippoglossoides). A large presence-only dataset is examined using multivariate kernel densities to define environmental envelopes, which we link to spatial distribution using a pan-Arctic oceanographic model. Occurrences generally fit the model well, although there were gaps in the predicted circum-Arctic distribution likely due to limited survey activity in many of the ice-covered seas around the Arctic Ocean. Bottom temperature and depth were major factors defining model fit to observations, but other factors, such as ecosystem interactions and larval drift could also influence distribution. Model predictions can be tested by increasing sampling effort in poorly explored regions and by studying the connectivity of putative populations. While abundances of Greenland halibut in the High Arctic are currently low, some areas are predicted to be suitable habitat for this species, suggesting that on-going sea-ice melt may lead to fisheries expansion into new areas.

Description: On the continental shelf, New Zealand bryozoans dominate ecologically-important three-dimensional benthic habitat providing structural complexity which hosts a wide variety of fauna, including economically valuable species (e.g. oysters and blue cod). The association between these species and bryozoan-dominated biogenic habitat commonly results in trawling damage to the benthos; eliminating pressure from destructive fishing practices could support bryozoan regrowth. In 2002, a voluntary fishing restriction was designated over part of one such bryozoan-dominated biogenic habitat, but the efficacy of this ban has not been assessed. Statutory marine protected areas have been proposed in the same area; they aim to achieve bryozoan protection and recovery. A comparison of the bryozoan fauna using benthic images from 2003 and 2019 from the Otago shelf, South Island, New Zealand/Te Waipounamu, Aotearoa shows that a 17-year voluntary fishing ban has been ineffective at increasing the cover and richness of bryozoan-dominated biogenic habitat on the Otago shelf. This study provides a baseline for future monitoring and develops useful metrics for recovery, while calling for further research on the biology and ecology of habitat-forming bryozoans.

Description: Ecosystem-based management is generally viewed as one of the most promising avenues for addressing the various anthropogenic pressures facing the world’s marine ecosystems. These approaches have been developed to varying degrees by individual countries or international organisations, but there remain a large proportion of marine ecosystems, particularly in developing regions, that have not yet been the subject of such research. In these areas, lack of effective regulation and the often high importance of the marine environment in providing food and economic opportunities, together create conditions where marine resources and habitats come under unsustainable levels of pressure. Here, we present a data-limited assessment approach to discern marine ecological patterns, in this case for the exclusive economic zone of Vietnam. By combining data from environmental and biological surveys from the Vietnamese national survey dataset and local oceanographic models, we have identified a series of 12 candidate ecological production units, delineated by their environmental characteristics, and the key commercial species that exist within them. These units are suggested as a possible foundation for a spatial management structure in the Vietnamese exclusive economic zone including considerations such as placement of marine protected areas, or ecological boundaries of key areas of socio-economic importance.

Description: This paper explores the possibility of using the ensemble modelling paradigm to fully capture assessment uncertainty and improve the robustness of advice provision. We identify and discuss advantages and challenges of ensemble modelling approaches in the context of scientific advice. There are uncertainties associated with every phase in the stock assessment process: data collection, assessment model choice, model assumptions, interpretation of risk, up to the implementation of management advice. Additionally, the dynamics of fish populations are complex, and our incomplete understanding of those dynamics and limited observations of important mechanisms, necessitate that models are simpler than nature. The aim is for the model to capture enough of the dynamics to accurately estimate trends and abundance, and provide the basis for robust advice about sustainable harvests. The status quo approach to assessment modelling has been to identify the “best” model and generate advice from that model, mostly ignoring advice from other model configurations regardless of how closely they performed relative to the chosen model. We discuss and make suggestions about the utility of ensemble models, including revisions to the formal process of providing advice to management bodies, and recommend further research to evaluate potential gains in modelling and advice performance.

Description: Climate effects on marine fish depend on life stage, particularly when life stages differ in habitat utilization. In the present study, we investigated life stage-dependent responses of lesser sandeel (Ammodytes marinus) to temperature at contrasting geographical scales. We related population density and individual growth to temperature and found different temperature responses between the first and the second years of life. During the first year of life, fish size was the single most important factor influencing sandeel abundances, indicating a positive relationship between growth and survival. In contrast, during the second year of life, autumn bottom temperature was negatively correlated with sandeel abundance, suggesting elevated mortality in warm years. Southerly areas, experiencing higher temperatures in general, were also the areas showing the strongest response to temperature. This study sheds light on how warming impacts population dynamics of one of the most important forage fishes in the North Sea and supports the discussion of underlying mechanisms.

Description: Summary Landslides can cause devastating damage. In particular, heavy rainfall-triggered landslides pose a chain of natural hazards. However, such events are often difficult to detect, leaving the physical processes poorly understood. Here we apply a novel surface-wave detector to detect and locate landslides during the transit of Typhoon Talas 2011. We identify multiple landslides triggered by Typhoon Talas, including a landslide in the Tenryu Ward, Shizuoka prefecture, Japan, ∼400 km east from the typhoon track. The Tenryu landslide displaced a total volume of 1.2 − −1.5 × 106 m. The landslide is much smaller than those detected by using globally recorded surface waves, yet the event generated coherent seismic signals propagating up to 3000 km away. Our observations show that attributes of small and large landslides may follow the same empirical scaling relationships, indicating possible invariant failure mechanisms. Our results also suggest an alerting technology to detect and locate landslides with a sparse seismic network.

Description: Summary We studied the broad-band spectra of the 8 largest earthquakes that have occurred in Chile in the last 25 years using strong-motion records and 1-Hz high-rate GNSS (cGNSS) data. To avoid the numerical instability problem with the double integration of the accelerograms, we computed velocity spectra integrating the acceleration time series in the spectral domain and compared them to time-differentiated the cGNSS displacement records. To compute the velocity spectrum, we used a multitaper algorithm so as to provide stability over the entire spectral band. We found that the velocity spectra of records obtained close to the main rupture of the earthquakes are different from classical Aki and Brune spectra. The velocity spectrum of large events in Chile presents a flat trend at low frequencies produced by the near-field waves. This trend converges at low frequencies to the static displacement as determined from GNSS data. For different magnitude earthquakes, we observe a transition in the ground-velocity spectrum from a decay of ${f^{ - 1}}$ at high frequencies and a flat trend at low frequencies to a more classical model with a peak at the corner frequency. The source-station distance influences the shape of the velocity spectrum at low frequencies, but there is no simple rule for the records available at present. At intermediate frequencies, the spectra are controlled by surface waves and S waves. We found a transition in the velocity spectrum for the 2014 Iquique earthquake, which indicates a change in the decay of the spectrum for stations at distances greater than ∼200 km. Finally, we show that the flat low-frequency trend of the velocity spectra determined from accelerograms, and the peak ground-displacement (PGD) determined from GNSS data scales with the moment to the power 2/3.

Description: Summary Seismic imaging techniques such as elastic full waveform inversion (FWI) have their spatial resolution limited by the maximum frequency present in the observed waveforms. Scales smaller than a fraction of the minimum wavelength cannot be resolved, and only a smoothed, effective version of the true underlying medium can be recovered. These finite-frequency effects are revealed by the upscaling or homogenization theory of wave propagation. Homogenization aims at computing larger scale effective properties of a medium containing small-scale heterogeneities. We study how this theory can be used in the context of FWI. The seismic imaging problem is broken down in a two-stage multiscale approach. In the first step, called homogenized full waveform inversion (HFWI), observed waveforms are inverted for a smooth, fully anisotropic effective medium, that does not contain scales smaller than the shortest wavelength present in the wavefield. The solution being an effective medium, it is difficult to directly interpret it. It requires a second step, called downscaling or inverse homogenization, where the smooth image is used as data, and the goal is to recover small-scale parameters. All the information contained in the observed waveforms is extracted in the HFWI step. The solution of the downscaling step is highly non-unique as many small-scale models may share the same long wavelength effective properties. We therefore rely on the introduction of external a priori information, and cast the problem in a Bayesian formulation. The ensemble of potential fine-scale models sharing the same long wavelength effective properties is explored with a Markov chain Monte Carlo algorithm. We illustrate the method with a synthetic cavity detection problem: we search for the position, size and shape of void inclusions in a homogeneous elastic medium, where the size of cavities is smaller than the resolving length of the seismic data. We illustrate the advantages of introducing the homogenization theory at both stages. In HFWI, homogenization acts as a natural regularization helping convergence toward meaningful solution models. Working with fully anisotropic effective media prevents the leakage of anisotropy induced by the fine scales into isotropic macro-parameters estimates. In the downscaling step, the forward theory is the homogenization itself. It is computationally cheap, allowing us to consider geological models with more complexity (e.g. including discontinuities) and use stochastic inversion techniques.

Description: Summary Particle Image Velocimetry (PIV), a method based on image cross-correlation, is widely used for obtaining velocity fields from time series of images of deforming objects. Rather than instantaneous velocities, we are interested in reconstructing cumulative deformation, and use PIV-derived incremental displacements for this purpose. Our focus is on analogue models of tectonic processes, which can accumulate large deformation. Importantly, PIV provides incremental displacements during analogue model evolution in a spatial reference (Eulerian) frame, without the need for explicit markers in a model. We integrate the displacements in a material reference (Lagrangian) frame, such that displacements can be integrated to track the spatial accumulative deformation field as a function of time. To describe cumulative, finite deformation, various strain tensors have been developed, and we discuss what strain measure best describes large shape changes, as standard infinitesimal strain tensors no longer apply for large deformation. PIV or comparable techniques have become a common method to determine strain in analogue models. However, the qualitative interpretation of observed strain has remained problematic for complex settings. Hence, PIV-derived displacements have not been fully exploited before, as methods to qualitatively characterize cumulative, large strain have been lacking. Notably, in tectonic settings, different types of deformation - extension, shortening, strike-slip - can be superimposed. We demonstrate that when shape changes are described in terms of Hencky strains, a logarithmic strain measure, finite deformation can be qualitatively described based on the relative magnitude of the two principal Hencky strains. Thereby, our method introduces a physically meaningful classification of large 2D strains. We show that our strain type classification method allows for accurate mapping of tectonic structures in analogue models of lithospheric deformation, and complements visual inspection of fault geometries. Our method can easily discern complex strike-slip shear zones, thrust faults and extensional structures and its evolution in time. Our newly developed software to compute deformation is freely available and can be used to post-process incremental displacements from PIV or similar autocorrelation methods.

Description: Sue Bowler remembers the Guernsey-born astronomer

Description: Jacqueline Mitton examines one strand in the life and research of the pioneering astronomer

Description: Robert Massey ponders the potential post-pandemic world for those working in astronomy and geophysics, following an online meeting in January

Description: A digest of some of the big stories from the past two months

Description: Summary It is generally accepted that melt extraction from the mantle at mid-ocean ridges is concentrated in narrow regions of elevated melt fraction called channels. Two feedback mechanisms have been proposed to explain why these channels grow by linear instability: shear flow of partially molten mantle and reactive flow of the ascending magma. These two mechanisms have been studied extensively, in isolation from each other, through theory and laboratory experiments as well as field and geophysical observations. Here, we develop a consistent theory that accounts for both proposed mechanisms and allows us to weigh their relative contributions. We show that interaction of the two feedback mechanisms is insignificant and that the total linear growth rate of channels is well-approximated by summing their independent growth rates. Furthermore, we explain how their competition is governed by the orientation of channels with respect to gravity and mantle shear. By itself, analysis of the reaction-infiltration instability predicts the formation of tube-shaped channels. We show that with the addition of even a small amount of extension in the horizontal, the combined instability favours tabular channels, consistent with the observed morphology of dunite bodies in ophiolites. We apply the new theory to mid-ocean ridges by calculating the accumulated growth and rotation of channels along streamlines of the solid flow. We show that reactive flow is the dominant instability mechanism deep beneath the ridge axis, where the most unstable orientation of high-porosity channels is sub-vertical. Channels are then rotated by the solid flow away from the vertical. The contribution of the shear-driven instability is confined to the margins of the melting region. Within the limitations of our study, the shear-driven feedback does not appear to be responsible for significant melt focusing or for the shallowly dipping seismic anisotropy that has been obtained by seismic inversions.

Description: Summary To understand earth processes, geoscientists infer subsurface earth properties such as electromagnetic resistivity or seismic velocity from surface observations of electromagnetic or seismic data. These properties are used to populate an earth model vector, and the spatial variation of properties across this vector sheds light on the underlying earth structure or physical phenomenon of interest, from groundwater aquifers to plate tectonics. However, to infer these properties the spatial characteristics of these properties need to be known in advance. Typically, assumptions are made about the length scales of earth properties, which are encoded a priori in a Bayesian probabilistic setting. In an optimisation setting, appeals are made to promote model simplicity together with constraints which keep models close to a preferred model. All of these approaches are valid, though they can lead to unintended features in the resulting inferred geophysical models owing to inappropriate prior assumptions, constraints or even the nature of the solution basis functions. In this work it will be shown that in order to make accurate inferences about earth properties, inferences can first be made about the underlying length scales of these properties in a very general solution basis. From a mathematical point of view, these spatial characteristics of earth properties can be conveniently thought of as “properties” of the earth properties. Thus, the same machinery used to infer earth properties can be used to infer their length scales. This can be thought of as an “infer to infer” paradigm analogous to the “learning to learn” paradigm which is now commonplace in the machine learning literature. However, it must be noted that (geophysical) inference is not the same as (machine) learning, though there are many common elements which allow for cross-pollination of useful ideas from one field to the other, as is shown here. A non-stationary trans-dimensional Gaussian Process (TDGP) is used to parameterise earth properties, and a multi-channel stationary TDGP is used to parameterise the length scales associated with the earth property in question. Using non-stationary kernels, i.e., kernels with spatially variable length scales, models with sharp discontinuities can be represented within this framework. As GPs are multi-dimensional interpolators, the same theory and computer code can be used to solve geophysical problems in 1D, 2D and 3D. This is demonstrated through a combination of 1D and 2D non-linear regression examples and a controlled source electromagnetic (CSEM) field example. The key difference between this and previous work using TDGP is generalised nested inference and the marginalisation of prior length scales for better posterior subsurface property characterisation.

Description: Summary The Z-transform of a complex time signal (or the analytic signal of a real signal) is equal to the Z-transform of a prediction error divided by the Z-transform of the prediction error operator. This inverse is decomposed into a sum of partial fractions, which are used to obtain impulse response operators formed by non-causal filters which complex-conjugate symmetric coefficients. The time-components are obtained by convolving the filters with the original signal, and the peak frequencies, corresponding to the poles of the prediction error operator, are used for mapping the time-components into frequency components. For non-stationary signals, this decomposition is done in sliding time windows, and the signal component values, in the middle of each window, are attributed to the peak value of its frequency response which corresponds to the pole of this partial fraction component. The result is an exact, but non-unique, time-frequency representation of the input signal. A sparse signal decomposition can be obtained by summing along the frequency axis in patches with similar characteristics in the time-frequency domain. The peak amplitude frequency of each new time component is obtained by computing a scalar prediction error operator in sliding time windows, resulting in a sparse time-frequency representation. In both cases, the result is a time-frequency matrix where an estimate of the frequency content of the input signal can be obtained by summation over the time variable. The performance of the new method is demonstrated with excellent results on a synthetic time signal, the LIGO gravitational wave signal, and on seismic field data.

Description: Summary Physical properties of near-surface soil and rock layers play a fundamental role in the seismic site effects analysis, being an essential element of seismic hazard assessment. Site-specific mechanical properties (i.e. shear- and compressional-wave velocities and mass density) can be inferred from surface wave dispersion and horizontal-to-vertical or ellipticity data by non-linear inversion techniques. Nevertheless, results typically exhibit significant inherent non-uniqueness as different models may fit the data equally well. Standard optimization inversion techniques minimize data misfit, resulting in a single representative model, rejecting other models providing similar misfit values. An alternative inversion technique can be formulated in the Bayesian framework, where the posterior probability density on the model space is inferred. This paper introduces an inversion approach of surface wave dispersion and ellipticity data based on a novel multizonal transdimensional Bayesian formulation. In particular, we parameterize one-dimensional layered velocity models by the varying number of Voronoi nuclei, allowing us to treat the number of layers as an unknown parameter of the inverse problem. The chosen parameterization leads to the transdimensional formulation of the model space, sampled by a reversible jump Markov chain Monte Carlo algorithm to provide an ensemble of random samples following the posterior probability density of model parameters. The used type of the sampling algorithm controls a model complexity (i.e. the number of layers) self-adaptively based on the measured data's information content. The method novelty lies in the parsimonious selection of sampling models and in the multizonal formulation of prior assumptions on model parameters, the latter allows including additional site-specific constraints in the inversion. These assumptions may be based on, e.g. stratigraphic logs, standard penetration tests, known water table, and bedrock depth. The multizonal formulation fully preserves the validity of the transdimensional one, as demonstrated analytically. The resultant ensemble of model samples is a discrete approximation of the posterior probability density function of model parameters and associated properties (e.g. VS30, quarter-wavelength average velocity profile, and theoretical SH-wave amplification function). Although the ultimate result is the posterior probability density function, some representative models are selected according to data fit and maximum of the posterior probability density function. We first validate our inversion approach based on synthetic tests and then apply it to field data acquired from the active seismic survey and single-station measurements of ambient vibrations at the SENGL seismic station site in central Switzerland.

Description: Volcán Quizapu, Chile, is an under-monitored volcano that was the site of two historical eruptions: an effusive eruption in 1846-1847 and a Plinian eruption in 1932, both of which discharged ∼5 km3 (DRE) of lava and/or tephra. The majority of material erupted in both cases is trachydacite, nearly identical for each event. We present H2O-saturated, phase equilibrium experiments on this end-member dacite magma, using a pumice sample from the 1932 eruption as the main starting material. At an oxygen fugacity (fO2) of ∼NNO+0.2, the phase assemblage of An25-30 plagioclase + amphibole + orthopyroxene, without biotite, is stable at 865±10 °C and 110±20 MPa H2O pressure (PH2O), corresponding to ∼4 km depth. At these conditions, experiments also reproduce the quenched glass composition of the starting pumice. At slightly higher PH2O and below 860 °C, biotite joins the equilibrium assemblage. Because biotite is not part of the observed Quizapu phase assemblage, its presence places an upper limit on PH2O. At the determined storage PH2O of ∼110 MPa, H2O undersaturation of the magma with XH20fluid==0.87 would align Ptotal to mineral-based geobarometry estimates of ∼130 MPa. However, XH20fluid=1〈 1 is not required to reproduce the Quizapu dacite phase assemblage and compositions. A second suite of experiments at lower fO2 shows that the stability fields of the hydrous silicates (amphibole and biotite) are significantly restricted at NNO-2 relative to NNO+0.2. Additional observations of Quizapu lava and pumice samples support the existing hypothesis that rapid pre-eruptive heating drove the effusive 1846-1847 eruption, with important refinements. We demonstrate that microlites in the end-member dacite lavas are consistent with in situ crystallization (during ascent), rather than transfer from an andesite. In one end-member dacite lava, newly identified reverse zoning in orthopyroxene and incipient destabilization of amphibole are consistent with small degrees of heating. Our work articulates a clear direction for future Quizapu studies, which are warranted given the active nature of the Cerro Azul-Descabezado Grande volcanic axis.

Description: Summary Until now, the polar motion resonance (PMR) complex frequency has been determined in the seasonal and retrograde diurnal band of the polar motion. In this study this resonance is studied in the prograde diurnal band, where polar motion is mainly composed of periodic terms caused by the diurnal oceanic tide. The resonance parameters (period and quality factor) are encompassed in the frequency transfer function between generating tidal potential and polar motion, and can be estimated accordingly. To this aim, we gather three published sets of prograde diurnal terms determined from GNSS and VLBI, to which we append our own estimates based upon a processing of the VLBI delays over the period 1990-2020. Then, by fitting the PMR parameters so that the prograde diurnal terms match the corresponding components of the tide generating potential, we obtained a resonance period of about 401 days and an equivalent quality factor of −22, differing from the ones reigning in the seasonal band (PPMR ≈ 431 days; QPMR ≈ 56 − 255) and in the retrograde diurnal band (PPMR ≈ 380 days; QPMR ≈ −10). Our estimates confirm strikingly the theoretical prediction derived from the tidal ocean angular momentum derived from the FES 2014 ocean tide model.

Description: Summary Bayesian inversion of electromagnetic data produces crucial uncertainty information on inferred subsurface resistivity. Due to their high computational cost, however, Bayesian inverse methods have largely been restricted to computationally expedient 1D resistivity models. In this study, we successfully demonstrate, for the first time, a fully 2D, trans-dimensional Bayesian inversion of magnetotelluric data. We render this problem tractable from a computational standpoint by using a stochastic interpolation algorithm known as a Gaussian process to achieve a parsimonious parametrization of the model vis-a-vis the dense parameter grids used in numerical forward modeling codes. The Gaussian process links a trans-dimensional, parallel tempered Markov chain Monte Carlo sampler, which explores the parsimonious model space, to MARE2DEM, an adaptive finite element forward solver. MARE2DEM computes the model response using a dense parameter mesh with resistivity assigned via the Gaussian process model. We demonstrate the new trans-dimensional Gaussian process sampler by inverting both synthetic and field magnetotelluric data for 2D models of electrical resistivity, with the field data example converging within 10 days on 148 cores, a non-negligible but tractable computational cost. For a field data inversion, our algorithm achieves a parameter reduction of over 32x compared to the fixed parameter grid used for the MARE2DEM regularized inversion. Resistivity probability distributions computed from the ensemble of models produced by the inversion yield credible intervals and interquartile plots that quantitatively show the non-linear 2D uncertainty in model structure. This uncertainty could then be propagated to other physical properties that impact resistivity including bulk composition, porosity and pore-fluid content.

Description: Summary Ambient wavefield data acquired on existing (so-called “dark fiber”) optical fiber networks using distributed acoustic sensing (DAS) interrogators allow users to conduct a wide range of subsurface imaging and inversion experiments. In particular, recorded low-frequency (

Description: Summary The stability of partial differential equations determines the properties of their solutions. This study focuses on the stability analysis of the equations describing wave propagation in fluids-saturated porous media. We briefly introduce the stability analysis method for the wave propagation equations and discuss the adverse effects on the solutions. In this way, the first part of this paper is mainly devoted to the analysis of the Tuncay and Corapcioglu's (TC) model, which describes the dynamic behaviour of porous media saturated with two immiscible fluids. It is pointed out that the TC model allows spatially bounded but time-exponentially exploding solutions and may yield unstable numerical results. Based on the deduced unstable factors, we construct a stable equivalent fluid (SEF) model. We rigorously analyze the stability of the SEF model using the energy method. For predicting the influence of saturation on wave velocity, the robustness of this model is preserved due to its consistency with the original TC model. Furthermore, the numerical simulations of the wave fields show that the results of the TC model exponentially increase with time after the initial effective wave signal, which does not occur in the SEF model curves. This indicates the necessity of considering the stability from a mathematical point of view during the construction of physical model. It could be useful to merge the mathematical stability theory with the geophysical wave propagation modelling theory.

Description: In the Southern Ribeira Belt (Brazil), the Itatins Complex, composed of ortho- and para-derived migmatites record granulite-facies metamorphism. Our work focuses on the ortho-derived migmatite, which displays features indicative of partial melting ranging from in-situ melting (cm-thick lenses and veins) to accumulations of melt as leucosomes and patches of diatexite. The unit comprises a biotite felsic granulite (metagranodiorite) with a residual neosome and a leucocratic neosome, generally referred to as leucosome. Phase equilibrium modelling using a modified bulk composition from the geochemical standard JG-1 (granodiorite) was carried out to evaluate whether a protolith of granodiorite bulk composition could produce the mineral assemblages observed in the residual neosome and the compositions of melt and residue produced by the partial melting. Isopleth thermobarometry of plagioclase, biotite and orthopyroxene points to P-T conditions of 0.7-1.0 GPa and 845-872 ºC. The modified granodiorite bulk composition generated just 10 mol % melt at these conditions, far lower than the amount estimated from outcrops (around 32 %). A T-XH2O pseudosection shows that increasing H2O content enhances the fertility of the bulk composition and it can become capable of producing melt volumes similar to that observed in outcrop. The melt compositions determined for partial melting of the modified granodiorite bulk composition in open- and closed-systems using THERMOCALC are granitic, and peraluminous. The melts have SiO2 contents between 52.02 mol % and 77.27 mol. % and show XFe values of 0.68 and 0.86. In K2O vs. Na2O+CaO, K2O vs. XFe and K2O-(FeO+MgO)-(Na2O+CaO) space natural samples (5 from the residual neosome and 2 from leucosome) from the Itatins Complex plot in the direction of the trends for melt and residue compositions obtained from modelling of the modified granodiorite bulk composition. The differences between modeled compositions and real rocks are interpreted to result from the interaction of melt with its residual counterpart. Chondrite-normalized REE patterns from the residual neosome show considerably greater depletion of HREE than geochemical standard JG-1 (granodiorite), and the two analyzed leucosomes show lower HREE contents. Zircon rims have REE patterns interpreted to indicate metamorphic recrystallization. U-Pb zircon Concordia ages indicate that the protolith formed during the Paleoproterozoic (2137 Ma) but that metamorphic recrystallization is Neoproterozoic (between 632 and 601 Ma). The metamorphic age pre-dates the intense granitic magmatism that affected the Southern Ribeira Belt between 600 and 570 Ma.

Description: Summary Uncovering the distribution of magnitudes and arrival times of aftershocks is a key to comprehending the characteristics of earthquake sequences, which enables us to predict seismic activities and conduct hazard assessments. However, identifying the number of aftershocks immediately after the main shock is practically difficult due to contaminations of arriving seismic waves. To overcome this difficulty, we construct a likelihood based on the detected data, incorporating a detection function to which Gaussian process regression (GPR) is applied. The GPR is capable of estimating not only the parameters of the distribution of aftershocks together with the detection function, but also credible intervals for both the parameters and the detection function. The property that the distributions of both the Gaussian process and aftershocks are exponential functions leads to an efficient Bayesian computational algorithm to estimate hyperparameters. After its validation through numerical tests, the proposed method is retrospectively applied to the catalog data related to the 2004 Chuetsu earthquake for the early forecasting of the aftershocks. The results show that the proposed method stably and simultaneously estimates distribution parameters and credible intervals, even within t ≤ 3h after the main shock.

Description: Ed Cliver, Lyndsay Fletcher and Hugh Hudson are looking for a photograph of Richard Carrington. Can you help?

Description: Richard Ellis recalls the life of the eminent cosmic-ray physicist

Description: Claire Davies recounts a successful outreach project based on Saturn and Jupiter's 2020 conjunction, a celebration of closeness in a year dominated by distancing

Description: Sumedh Anathpindika reviews some recent results that shed new light on the dynamical evolution of giant molecular clouds and discusses the impact of ambient environment on their ability to form stars

Description: Leslie Morrison and Catherine Hohenkerk consider the future direction of the leap second

Description: An RAS Specialist Discussion Meeting organized by Malcolm Druett and Ben Snow on 13 November 2020 drew a broad international audience to explore a tricky layer of the Sun

Description: Summary We reconstruct the post-52 Ma seafloor spreading history of the Southwest Indian Ridge at 44 distinct times from inversions of ≈20,000 magnetic reversal, fracture zone, and transform fault crossings, spanning major regional tectonic events such as the Arabia-Eurasia continental collision, the Arabia Peninsula’s detachment from Africa, the arrival of the Afar mantle plume below eastern Africa, and the initiation of rifting in eastern Africa. Best-fitting and noise-reduced rotation sequences for the Nubia-Antarctic, Lwandle-Antarctic, and Somalia-Antarctic plate pairs indicate that spreading rates everywhere along the ridge declined gradually by ≈50 percent from ≈31 Ma to 19-18 Ma. A concurrent similar-magnitude slowdown in the component of the Africa plate’s absolute motion parallel to Southwest Indian Ridge spreading suggests that both were caused by a 31-18 Ma change in the forces that drove and resisted Africa’s absolute motion. Possible causes for this change include the effects of the Afar mantle plume on eastern Africa or the Arabia Peninsula’s detachment from the Somalia plate, which culminated at 20-18 Ma with the onset of seafloor spreading in the Gulf of Aden. At earlier times, an apparently robust but previously unknown ≈6-Myr-long period of rapid kinematic change occurred from 43 Ma to 37 Ma, consisting of a ≈50 percent spreading rate slowdown from 43-40 Ma followed by a full spreading rate recovery and 30-40 ○ clockwise rotation of the plate slip direction from 40-37 Ma. Although these kinematic changes coincided with a reconfiguration of the paleoridge geometry, their underlying cause is unknown. Southwest Indian Ridge abyssal hill azimuths are consistent with the slip directions estimated with our newly derived Somalia-Antarctic and Lwandle-Antarctic angular velocities, adding confidence in their reliability. Lwandle-Antarctica plate motion has closely tracked Somalia-Antarctic plate motion since 50 Ma, consistent with slow-to-no motion between the Lwandle and Somalia plates for much of that time. In contrast, Nubia-Somalia rotations estimated from our new Southwest Indian Ridge rotations indicate that 189±34 km of WNW-ESE divergence between Nubia and Somalia has occurred in northern Africa since 40 Ma, including 70-80 km of WNW-ESE divergence since 17-16 Ma, slow to no motion from 26-17 Ma, and 109±38 km of WNW-ESE divergence from 40 Ma to ≈26 Ma absent any deformation within eastern Antarctica before 26 Ma.

Description: Based on the linear traveltime interpolation (LTI) algorithm, we propose an improved LTI (referred as ILTI) algorithm for multiphase seismic ray tracing, which uses a velocity node in model parameterization and introduces secondary nodes between adjacent velocity nodes. To better fit the undulating surface model, an irregular velocity node is used in near the irregular interface, and regular velocity nodes are still used in the region far away from the irregular interface. We derive an iterative fixed-point formula for calculating traveltime. By combining multistage computational technology and wavefront narrowband expansion technology, the proposed ILTI algorithm can efficiently trace the multiphase seismic raypath and compute the corresponding traveltime field. Through comparison and analysis with the traditional LTI algorithm, its computational accuracy can be highlighted by at least one order of magnitude. Compared with the popular fast marching method (FMM) and irregular shortest-path (ISPM) algorithms, it also has the advantages in terms of computational accuracy and efficiency. Numerical simulations in the Marmousi model show that the algorithm is also suitable for tracking multiphase seismic rays in the complex velocity model.

Description: Angle domain common image gathers (ADCIGs), commonly regarded as important prestacked gathers, provide the information required for velocity model construction and the phase and amplitude information needed for subsurface structures in oil/gas exploration. Based on the constant-density acoustic-wave equation assumption, the ADCIGs generated from reverse time migration ignore the fact that the subsurface density varies with location. Consequently, the amplitude versus angle (AVA) analysis extracted from these ADCIGs is not accurate. To partially solve this problem and to improve the accuracy of the AVA analysis, we developed amplitude-preserving ADCIGs suitable for density variations with the assumption of acoustic approximation. The Poynting vector approach, which is efficient and computationally inexpensive, was used to calculate the high-resolution wavefield propagation. The ADCIGs generated from the velocity and density perturbations match the theoretical AVA relationship better than ADCIGs with constant density. The extraction of the AVA analysis of the various combinations of the subsurface medium indicates that the density is non-negligible, especially when the density contrast is sharp. Numerical examples based on a layered model verify our conclusions.

Description: Conventional triaxial loading tests with different confining pressures and stress-seepage coupling tests on sandstone with different confining pressures and seepage pressures were conducted. A permeability model considering strength and strain was established, which better characterized the progressive deformation mechanical behaviour of sandstone under stress-seepage coupling. The results showed the following. (i) The confining pressure not only affects the peak strength of sandstone but also affects the axial deformation under conventional triaxial loading conditions. (ii) Compared with the seepage pressure effect, the degree of the confining pressure effect on the strength of sandstone was weaker, but the degree of that on the axial, radial and volumetric deformations of sandstone was stronger under stress-seepage coupling. (iii) With increasing confining pressure, the axial strain of sandstone decreased, while the corresponding radial and volumetric strains showed progressively increasing evolution characteristics under identical seepage pressures and different confining pressures. With increasing seepage pressure, the axial strain continuously decreased, while the corresponding radial and volumetric strains showed the progressive evolution characteristic of first increasing and then decreasing under identical confining pressures and different seepage pressures. (iv) Compared with the confining pressure effect, the degree of the seepage pressure effect on the permeability progressive evolution law of sandstone was weaker under stress-seepage coupling. The research conclusions could enrich the theories for the prevention and control of water inrush accidents in coal mines.

Description: The mantle-crust boundary beneath oceanic spreading centres is a major chemical and thermal interface on Earth. Observations in ophiolites reveal that it is underlined by a dunitic transition zone (DTZ) that can reach a few hundred meters in thickness and hosting abundant chromitite ore bodies. The dunites have been deciphered as essentially mantle-derived in most ophiolitic massifs, i.e. reactional residues of interactions between peridotite and percolating melt(s). Although both dunite and chromitite in ophiolites have been the focus of many studies, the reasons of their systematic association remain unclear. In this study we have explored the inclusion content of the chromite grains disseminated in the dunites from the DTZ exposed in the Maqsad area of the Oman ophiolite where a former asthenospheric diapir is exposed. Similarly to chromite in chromitite ore bodies, disseminated chromite grains in dunites contain a great diversity of silicate inclusions. Based on the major and minor element composition of 1794 single silicate inclusions in chromites from 285 samples of dunite and associated rocks in the DTZ, we infer that the disseminated chromites formed by a similar “metallogenic” process to the chromitites, and that, as a whole, dunites from the DTZ actually represent the low grade endmember of a single, giant ore body. The nature of the silicate inclusions (amphibole and mica among others) enclosed in chromite grains in dunites from the Maqsad DTZ precludes their crystallization from an anhydrous primitive basaltic melt, and rather calls for a crystallization from a melt hybrid between common mafic melts and more exotic Si-, Na- and volatile-rich fluids. The hybrid parent medium of both dunites and chromitites results from the interaction between an asthenospheric diapir (the MORB source), and a colder, altered lithospheric lid and hydrothermal fluids responsible for this alteration. The excess silica in the hybrid melt is provided by the incongruent dissolution of enstatite from mantle harzburgite and/or from moderate degree of partial melting of the altered gabbroic crust. The chemical composition of the silicate inclusions is more variable when enclosed in the disseminated chromites than in the chromitites, suggesting a greater variability of melts and/or fluids fractions involved in the genesis of dunites than of chromite ores. Finally, the DTZ can be viewed as a metamorphic contact aureole between episodically rising asthenospheric diapirs and formerly accreted axial lithospheric lids. Our conclusion about the chicken and egg dilemma linking dunites and chromitites beneath oceanic spreading centres (i.e. Do the chromitites form in response to the formation of dunites or conversely?) is that the mantle dunitization itself is a potential way for the release of Cr and its re-concentration as chromite ores, and that in turn the competition between orthopyroxene (+/- plagioclase) and chromite fractionation during this fluid/melt/peridotite reactional process is responsible of the great mineralogical and chemical variability of the DTZ dunites.

Description: Summary Most of the existing three-dimensional (3-D) electromagnetic (EM) modeling solvers based on the integral equation (IE) method exploit fast Fourier transform (FFT) to accelerate the matrix-vector multiplications. This in turn requires a laterally-uniform discretization of the modeling domain. However, there is often a need for multi-scale modeling and inversion, for instance, to properly account for the effects of non-uniform distant structures, and at the same time, to accurately model the effects from local anomalies. In such scenarios, the usage of laterally-uniform grids leads to excessive computational loads, both in terms of memory and time. To alleviate this problem, we developed an efficient 3-D EM modeling tool based on a multi-nested IE approach. Within this approach, the IE modeling is first performed at a large domain and on a (laterally-uniform) coarse grid, and then the results are refined in the region of interest by performing modeling at a smaller domain and on a (laterally-uniform) denser grid. At the latter stage, the modeling results obtained at the previous stage are exploited. The lateral uniformity of the grids at each stage allows us to keep using the FFT for the matrix-vector multiplications. An important novelty of the paper is a development of a “rim domain” concept which further improves the performance of the multi-nested IE approach. We verify the developed tool on both idealized and realistic 3-D conductivity models, and demonstrate its efficiency and accuracy.

Description: The Miocene reservoirs in prolific Krishna-Godavari basin are mostly fluvial deposits and laminated or blocky in nature. The type of reservoir quality depends on associated geological environments. Due to several lateral variations in reservoir properties, a similar kind of workflow for reservoir characterisation does not work. Customised workflow needs to be applied in this area for estimation of petrophysical properties or rock physical analysis for reservoir quality prediction. As the major input of rock physical analysis is petrophysical properties, it is crucial to estimate these properties accurately. Meanwhile, it is also important to check the seismic sensitivity to change in fluid saturation in the reservoir characterisation process. The analysis assures the presence of reservoir and hydrocarbon contact in seismic sensitivity, which is essential for removing risk. Integrating the geological model with rock physical analysis for reservoir characterisation at the drilled well, the reservoir quality at undrilled prospects is predicted. In this study, the comprehensive study for reservoir characterisation of Miocene reservoirs consists of three different steps: calculation of petrophysical properties for mixed of thick and laminated sequence, rock physical analysis for identification of hydrocarbon reservoir and corresponding seismic sensitivity for change in saturation and finally the rock physics template for prediction of reservoir quality away from the drilled well. Results from the study have added significant value in de-risking the number of undrilled prospects in this area.

Description: We present a three-dimensional (3D) general-measure inversion scheme of crosswell electromagnetic (EM) data in the frequency domain with a direct forward solver. In the forward problem, we discretised the EM Helmholtz equation by the staggered-grid finite difference (SGFD) scheme and solved it using the Intel MKL PARDISO direct solver. By applying a direct solver, we simultaneously solved the multisource forward problems at a given frequency. In the inversion, we integrated a general measure of data misfit and model constraints with linearised least-squares inversion. We reconstructed a model with blocky features by selecting the appropriate measure parameters and model constraints. We used the adjoint equation method to explicitly calculate the Jacobian matrix, which facilitated the determination of an appropriate initial value for the regularisation coefficient in the objective function. We illustrated the inversion scheme using synthetic crosswell EM data with a general measure, the L2 norm, and, specifically, two mixed norms.

Description: Summary Denoising and onset time picking of signals are essential before extracting source information from collected seismic/microseismic data. We proposed an advanced deep dual-tasking network (DDTN) that integrates these two procedures sequentially to achieve the optimal performance. Two homo-structured encoder-decoder networks with specially designed structures and parameters are connected in series for handling the denoising and detection of microseismic signals. Based on the similarity of data types, the output of the denoising network will be imported into the detection network to obtain labels for the signal duration. The procedures of denoising and duration detection can be completed in an integrated way, where the denoised signals can improve the accuracy of onset time picking. Results show that the method has a good performance for the denoising of microseismic signals that contain various types and intensities of noise. Compared with existing methods, DDTN removes the noise with a minor waveform distortion. It is ideal for recovering the microseismic signal while maintaining a good capacity for onset time picking when the signal-to-noise ratio is low. Based on that, the second network can detect a more accurate duration of microseismic signals and thus obtain more accurate onset time. The method has great potential to be extended to the study of exploration seismology and earthquakes.

Description: Summary The ongoing InSight mission has recently deployed very broad band seismometers to record the Martian seismic activity. These recordings constitute the first seismic data set collected at the surface of Mars. This unique but sparse record compels for the development of new techniques tailored to make the best use of the specific context of single station-multiple events with several possible ranges of uncertainties on the event location. To this end, we conducted sets of Markov chain Monte-Carlo inversions for the 1-D seismic structure of Mars. We compared two inversion techniques that differ from the nature of the parameterization on which they rely. A first classical approach based on a parameterization of the 1-D seismic profile using Bézier curves. A second, less conventional approach that relies on a parameterization in terms of quantities that influence the thermo-chemical evolution of the planet (mantle rheology, initial thermal state, and composition), which accounts for 4.5 Gyr of planetary evolution. We considered several combinations of true model parameters to retrieve, and explored the influence of the type of seismic data (body waves with or without surface waves), the number of events and their associated epicentral distances and uncertainties, and the presence of potential constraints on Moho depth inferred from independent measurements/considerations (receiver functions and gravity data). We show that due to its inherent tighter constraints the coupled approach allows a considerably better retrieval of Moho depth and the seismic structure underneath it than the classical inversion, under the condition that the physical assumptions made in coupled approach are valid for Mars. In addition, our tests indicate that in order to constrain the seismic structure of Mars with InSight data, the following independent conditions must be met: (1) The presence of surface waves triggered by an internal source to constrain the epicentral distance. (2) The presence of just a few well-localized impact sources, with at least one located at close epicentral distance (

Description: Summary In the field of seismic interferometry, cross-correlations are used to extract Green’s function from ambient noise data. By applying a single station variation of the method, using auto-correlations, we are in principle able to retrieve zero-offset reflections in a stratified Earth. These reflections are valuable as they do not require an active seismic source and, being zero-offset, are better constrained in space than passive earthquake based measurements. However, studies that target Moho signals with ambient noise auto-correlations often give ambiguous results with unclear Moho reflections. Using a modified processing scheme and phase-weighted stacking, we determine the Moho P wave reflection time from vertical auto-correlation traces for a test station with a known simple crustal structure (HYB in Hyderabad, India). However, in spite of the simplicity of the structure, the auto-correlation traces show several phases not related to direct reflections. Although we are able to match some of these additional phases in a qualitative way with synthetic modelling, their presence makes it hard to identify the reflection phases without prior knowledge. This prior knowledge can be provided by receiver functions. Receiver functions (arising from mode conversions) are sensitive to the same boundaries as auto-correlations, so should have a high degree of comparability and opportunity for combined analysis but in themselves are not able to independently resolve VP, VS, and Moho depth. Using the timing suggested by the receiver functions as a guide, we observe the Moho S wave reflection on the horizontal auto-correlation of the north component but not on the east component. The timing of the S reflection is consistent with the timing of the PpSs-PsPs receiver function multiple, which also depends only on the S velocity and Moho depth. Finally, we combine P receiver functions and auto-correlations from HYB in a depth-velocity stacking scheme that gives us independent estimates for VP, VS, and Moho depth. These are found to be in good agreement with several studies that also supplement receiver functions to obtain unique crustal parameters. By applying the auto-correlation method to a portion of the EASI transect crossing the Bohemian Massif in central Europe, we find approximate consistency with Moho depths determined from receiver functions and spatial coherence between stations, thereby demonstrating that the method is also applicable for temporary deployments. Although application of the auto-correlation method requires great care in phase identification, it has the potential to resolve both average crustal P and S velocities alongside Moho depth in conjunction with receiver functions.

Description: Summary By combining scaled laboratory experiments and numerical simulations, this study presents a quantitative analysis of the bending radius (RB) of subducting slabs within the upper mantle, taking into account the effects of age (A). Based on a half-space cooling model, we constrain the density (ρ), viscosity (η) and thickness (h) of slabs as a function of A, and develop representative models to estimate RB for different A. Laboratory subduction models produce visually contrasting bending curvatures for young (A = 10 Ma), intermediate (A = 70 Ma) and old (A = 120 Ma) slabs. Young slabs undergo rollback, resulting in a small bending radius (scaled up RB ∼ 150 km), whereas old slabs subduct along a uniformly dipping trajectory with large bending radius (RB ∼ 500 km). Equivalent real scale computational fluid dynamic (CFD) simulations reproduce similar bending patterns of the subducting slabs, and yield RB versus A relations fairly in agreement with the laboratory results. We balance the buoyancy driven bending, flexural-resistive moments and viscous flow induced suction moment to theoretically evaluate the rate of slab bending. The analytical solution suggests an inverse relation of the bending rate with A, which supports our experimental findings. Finally, slab geometries of selected natural subduction zones, derived from high-resolution seismic tomographic images have been compiled to validate the experimental RB versus A regression. We also discuss the subduction settings in which this regression no longer remains valid.

Description: Summary As recovering the crust-mantle/Moho density contrast (MDC) significantly depends on the properties of the Earth's crust and upper mantle, varying from place to place, it is an oversimplification to define a constant standard value for it. It is especially challenging in Antarctica, where almost all the bedrock is covered with a thick layer of ice, and seismic data cannot provide a sufficient spatial resolution for geological and geophysical applications. As an alternative we determine the MDC in Antarctica and its surrounding seas with a resolution of 1° × 1° by the Vening Meinesz-Moritz gravimetric-isostatic technique using the XGM2019e Earth Gravitational Model and Earth2014 topographic/bathymetric information along with CRUST1.0 and CRUST19 seismic crustal models. The numerical results show that our model, named HVMDC20, varies from 81 kg/m3 in the Pacific Antarctic mid-oceanic ridge to 579 kg/m3 in the Gamburtsev Mountain Range in the central continent with a general average of 403 kg/m3. To assess our computations, we compare our estimates with those of some other gravimetric as well as seismic models (KTH11, GEMMA12C, KTH15C, and CRUST1.0), illustrating that our estimates agree fairly well with KTH15C and CRUST1.0 but rather poor with the other models. In addition, we compare the geological signatures with HVMDC20, showing how the main geological structures contribute to the MDC. Finally, we study the remaining glacial isostatic adjustment effect on gravity to figure out how much it affects the MDC recovery, yielding a correlation of the optimum spectral window (7≤ n ≤12) between XGM2019e and W12a GIA model of the order of ∼ 0.6 contributing within a negligible $pm 14$ kg/m3 to the MDC.

Description: Summary Spectral information obtained from induced polarization (IP) measurements can be used in a variety of applications and is often gathered in frequency domain (FD) at the laboratory scale. In contrast, field IP measurements are mostly done in time domain (TD). Theoretically, the spectral content from both domains should be similar. In practice, they are often different, mainly due to instrumental restrictions as well as the limited time and frequency range of measurements. Therefore, a possibility of transition between both domains, in particular for the comparison of laboratory FD IP data and field TD IP results, would be very favourable. To compare both domains, we conducted laboratory IP experiments in both TD and FD. We started with three numerical models and measurements at a test circuit, followed by several investigation at different wood and sandstone samples. Our results demonstrate that the differential polarizability (DP), which is calculated from the TD decay curves, can be compared very well with the phase of the complex electrical resistivity. Thus, DP can be used for a first visual comparison of FD and TD data, which also enables a fast discrimination between different samples. Furthermore, to compare both domains qualitatively, we calculated the relaxation time distribution (RTD) for all data. The results are mostly in agreement between both domains, however, depending on the TD data quality. It is striking that the DP and RTD results are in better agreement for higher data quality in TD. Nevertheless, we demonstrate that IP laboratory measurements can be carried out in both TD and FD with almost equivalent results. The RTD enables a good comparability of FD IP laboratory data with TD IP field data.

Description: Summary Isostasy explains why observed gravity anomalies are generally much weaker than what is expected from topography alone, and why planetary crusts can support high topography without breaking up. On Earth, it is used to subtract from gravity anomalies the contribution of nearly compensated surface topography. On icy moons and dwarf planets, it constrains the compensation depth which is identified with the thickness of the rigid layer above a soft layer or a global subsurface ocean. Classical isostasy, however, is not self-consistent, neglects internal stresses and geoid contributions to topographical support, and yields ambiguous predictions of geoid anomalies. Isostasy should instead be defined either by minimizing deviatoric elastic stresses within the silicate crust or icy shell, or by studying the dynamic response of the body in the long-time limit. In this paper, I implement the first option by formulating Airy isostatic equilibrium as the linear response of an elastic shell to a combination of surface and internal loads. Isostatic ratios are defined in terms of deviatoric Love numbers which quantify deviations with respect to a fluid state. The Love number approach separates the physics of isostasy from the technicalities of elastic-gravitational spherical deformations, and provides flexibility in the choice of the interior structure. Since elastic isostasy is invariant under a global rescaling of the shell shear modulus, it can be defined in the fluid shell limit, which is simpler and reveals the deep connection with the asymptotic state of dynamic isostasy. If the shell is homogeneous, minimum stress isostasy is dual to a variant of elastic isostasy called zero deflection isostasy, which is less physical but simpler to compute. Each isostatic model is combined with general boundary conditions applied at the surface and bottom of the shell, resulting in one-parameter isostatic families. At long wavelength, the thin shell limit is a good approximation, in which case the influence of boundary conditions disappears as all isostatic families members yield the same isostatic ratios. At short wavelength, topography is supported by shallow stresses so that Airy isostasy becomes similar to either pure top loading or pure bottom loading. The isostatic ratios of incompressible bodies with three homogeneous layers are given in analytical form in the text and in complementary software.

Description: This study aims for the protection of a crude-oil pipeline, buried at a shallow depth, against a probable environmental hazard and pilferage. Both surface and borehole geophysical techniques such as electrical resistivity tomography (ERT), ground penetrating radar (GPR), surface seismic refraction tomography (SRT), cross-hole seismic tomography (CST) and cross-hole seismic profiling (CSP) were used to map the vulnerable zones. Data were acquired using ERT, GPR and SRT along the pipeline for a length of 750 m, and across the pipeline for a length of 4096 m (over 16 profiles of ERT and SRT with a separation of 50 m) for high-resolution imaging of the near-surface features. Borehole techniques, based on six CSP and three CST, were carried out at potentially vulnerable locations up to a depth of 30 m to complement the surface mapping with high-resolution imaging of deeper features. The ERT results revealed the presence of voids or cavities below the pipeline. A major weak zone was identified at the central part of the study area extending significantly deep into the subsurface. CSP and CST results also confirmed the presence of weak zones below the pipeline. The integrated geophysical investigations helped to detect the old workings and a deformation zone in the overburden. These features near the pipeline produced instability leading to deformation in the overburden, and led to subsidence in close vicinity of the concerned area. The area for imminent subsidence, proposed based on the results of the present comprehensive geophysical investigations, was found critical for the pipeline.

Description: Existing saturation models cannot effectively describe the specific fractured-vuggy carbonate reservoir in area A of the Sichuan Basin, southwestern China. This reservoir has got a wide pore size distribution, strong heterogeneity, high gas saturation and complex electrical conductivity mechanism. Hence, the present study attempted to establish a new saturation calculation equation for this carbonate reservoir based on the microscopic conductivity mechanism of the rock. Here, we first used the multiscale computed tomography (CT) scanning method to build multiscale digital rocks. Subsequently, we applied the maximum sphere algorithm to extract the pore space structure and constructed the multiscale pore network models. By using the cross-scale fusion method, four different pore configurations were determined. Then, the percolation theory was implemented to simulate the conductivity mechanism of the constructed pore network models. As a result, the fluid distribution characteristics and the resistivity variation trends of the different pore structures were obtained. The simulation results showed that the fracture system of the studied reservoir had a much greater effect than the vug system on the carbonate rock's electrical conductivity, and the conductivity was closely related to the fluid distribution. In addition, based on the simulation results, a new conductivity model was proposed that incorporates the coupling phenomenon of pores, vugs and fractures; and also a new saturation calculation equation for triple-porosity media was established. The observations indicated that the field application of the proposed equation had an acceptable performance with an error value of less than 2.56%. The results from the present study provide new insights into the evolution of electrical properties in triple-porosity carbonate systems.

Description: Summary The Pollino range is a region of slow deformation where earthquakes generally nucleate on low-angle normal faults. Recent studies have mapped fault structures and identified fluid-related dynamics responsible for historical and recent seismicity in the area. Here, we apply the coda-normalization method at multiple frequencies and scales to image the 3D P-wave attenuation (QP) properties of its slowly-deforming fault network. The wide-scale average attenuation properties of the Pollino range are typical for a stable continental block, with a dependence of QP on frequency of $Q_P^{-1}=(0.0011pm 0.0008) f^{(0.36pm 0.32)}$. Using only waveforms comprised in the area of seismic swarms, the dependence of attenuation on frequency increases ($Q_P^{-1}=(0.0373pm 0.0011) f^{(-0.59pm 0.01)}$), as expected when targeting seismically-active faults. A shallow very-low-attenuation anomaly (max depth of 4-5 km) caps the seismicity recorded within the western cluster 1 of the Pollino seismic sequence (2012, maximum magnitude MW = 5.1). High-attenuation volumes below this anomaly are likely related to fluid storage and comprise the western and northern portions of cluster 1 and the Mercure basin. These anomalies are constrained to the NW by a sharp low-attenuation interface, corresponding to the transition towards the eastern unit of the Apennine Platform under the Lauria mountains. The low-seismicity volume between cluster 1 and cluster 2 (maximum magnitude MW = 4.3, east of the primary) shows diffuse low-to-average attenuation features. There is no clear indication of fluid-filled pathways between the two clusters resolvable at our resolution. In this volume, the attenuation values are anyway lower than in recognized low-attenuation blocks, like the Lauria Mountain and Pollino Range. As the volume develops in a region marked at surface by small-scale cross-faulting, it suggests no actual barrier between clusters, more likely a system of small locked fault patches that can break in the future. Our model loses resolution at depth, but it can still resolve a 5-to-15-km-deep high-attenuation anomaly that underlies the Castrovillari basin. This anomaly is an ideal deep source for the SE-to-NW migration of historical seismicity. Our novel deep structural maps support the hypothesis that the Pollino sequence has been caused by a mechanism of deep and lateral fluid-induced migration.

Description: A large compilation of quality-curated major and trace element data has been assembled to investigate how trace element patterns of mafic and ultramafic magmas have varied with time through particular settings from the Archean to the Phanerozoic, the primary objective being to recognise at what times particular patterns of variation emerge, and how similar these are to baseline data sets representing tectonic settings in the modern Earth. The most informative element combinations involve Nb, Th and the REE, where REE are represented by “lambda” parameters describing slope and shape of patterns. Combinations of the ratios of Th, Nb, La and lambda values from Archean and early Proterozoic basalts and komatiites reveal a distinctive pattern that is common in most well-sampled terranes, defining a roughly linear trend in multi-dimensional space from compositions intermediate between modern n-MORB and primitive mantle at one end, towards compositions approximating middle-to-upper continental crust at the other. We ascribe this “Variable Th/Nb” trend in most instances to varying degrees of crustal contamination of magmas with similar compositions to modern oceanic plateau basalts. Komatiites had slightly more depleted sources than basalts, consistent with the hypothesis of derivation from plume tails and heads, respectively. The most significant difference between Precambrian and Phanerozoic plume-derived basalts is that the distinctive OIB-like enriched source component appears to be largely missing from the Archean and Proterozoic geologic record, although isolated examples of OIB-like trace element characteristics are evident in datasets from even the oldest preserved greenstones. Phanerozoic intra-cratonic LIPs, such as the 260 Ma Emeishan in China, have fundamentally different geochemical characteristics to Archean and Paleoproterozoic assemblages; the oldest Proterozoic LIP we have identified that has this type of “modern” signal is the Midcontinent Rift at 1100 Ma. The data are consistent with plume tail sources having changed from being dominantly depleted in the Archean Earth to dominantly enriched in the Phanerozoic Earth, while plume head sources have hardly changed at all. Trace element patterns considered to be diagnostic of subduction are locally present but rare in Archean terranes and become more prevalent through the Proterozoic, although this conclusion is tempered by the large degree of overlap in compositional space between continental arc magmas and continental flood basalts. This overlap reflects the difficulty of distinguishing the effects of supra-subduction metasomatism and flux melting from those of crustal contamination. Additional factors must also be borne in mind, particularly that trace element partitioning systematics may have been different in all environments in a hotter planet, and large-scale asthenospheric overturns might have been predominant over modern-style plumes in the Archean Earth. Some basaltic suites in particular Archean terranes, notably the western parts of both the Yilgarn and Pilbara cratons in Western Australia and parts of the Superior Craton, have restricted but locally predominant suites of basalts with characteristics akin to modern oceanic arcs, suggesting that some process similar to modern subduction was preserved in these particular belts. Ferropicrite magmas with distinctive characteristics typical of modern OIBs and some continental LIPs (notably Emeishan) are rare but locally predominant in some Archean and early Proterozoic terranes, implying that plume sources were beginning to be fertilised by enriched, probably subducted, components as far back as the Mesoarchean. We see no evidence for discontinuous secular changes in mantle-derived magmatism with time that could be ascribed to major mantle reorganisation events. The Archean-Proterozoic transition appears to be entirely gradational from this standpoint. The transition from Archean-style to Phanerozoic-style plume magmatism took place somewhere between 1900 Ma (age of the Circum-Superior komatiitic basalts suites) and 1100 Ma (age of the Midcontinent Rift LIP).

Description: Summary Perturbation method applied for the phase velocities in the monoclinic model with a horizontal symmetry plane gives insight into the seismic signatures of P, S1 and S2 wave modes associated with the monoclinic stiffness coefficients. The perturbation-based approximations are very accurate and can be used for modeling and inversion purpose.

Description: Anna Ridley describes a new exhibition at Old Government House, Sydney, that uses documents from the Royal Astronomical Society library and archive – and technology – to help tell the story of scientific endeavour in colonial New South Wales

Description: Summary We present a novel approach for imaging global mantle discontinuities based on full-waveform inversion (FWI). Over the past decades, extensive research has been done on imaging mantle discontinuities at approximately 400 km and 670 km depth. Accurate knowledge of their topography can put strong constraints on thermal and compositional variations and hence geodynamic modelling. So far, however, there is little consensus on their topography. We present an approach based on adjoint tomography, which has the advantage that Fréchet derivatives for discontinuities and measurements, to be inverted for, are fully consistent. Rather than working with real data, we focus on synthetic tests, where the answer is known in order to be able to evaluate the performance of the developed method. All calculations are based on the community code SPECFEM3D_GLOBE. We generate data in fixed 1-D or 3-D elastic background models of mantle velocity. Our ‘data’ to be inverted contain topography along the 400 km and 670 km mantle discontinuities. To investigate the approach, we perform several tests: (i) In a situation where we know the elastic background model 1-D or 3-D, we recover the target topography fast and accurately, (ii) The exact misfit is not of great importance here, except in terms of convergence speed, similar to a different inverse algorithm, (iii) In a situation where the background model is not known, the convergence is markedly slower, but there is reasonable convergence towards the correct target model of discontinuity topography. It has to be noted that our synthetic test is idealised and in a real data situation, the convergence to and uncertainty of the inferred model is bound to be larger. However, the use of data consistent with Fréchet kernels seems to pay off and might improve our consensus on the nature of mantle discontinuities. Our workflow could be incorporated in future FWI mantle models to adequately infer boundary interface topography.

Description: Summary This study investigates the seismic structure and anisotropy in the crust beneath Madagascar and south-eastern Africa, using receiver functions. The understanding of seismic anisotropy is essential for imaging past and present deformation in the lithosphere-asthenosphere system. In the upper mantle, seismic anisotropy mainly results from the orientation of olivine, which deforms under tectonic (fossil anisotropy) or flow processes (in the asthenosphere). In the crust, the crystallographic alignment of amphiboles, feldspars(plagioclase) or micas or the alignment of heterogeneities such as fractures, add to a complex geometry, which results in challenges to understanding the Earth's shallow structure. The decomposition of receiver functions into back-azimuth harmonics allows to characterize orientations of lithospheric structure responsible for azimuthally-varying seismic signals, such as a dipping isotropic velocity contrasts or layers of azimuthal seismic anisotropy. By analysing receiver function harmonics from records of 48 permanent or temporary stations this study reveals significant azimuthally-varying signals within the upper crust of Madagascar and south-eastern Africa. At 30 stations crustal anisotropy dominates the harmonics while the signature of a dipping isotropic contrast is dominant at the remaining 18 stations. However, all stations’ back-azimuth harmonics show complex signals involving both dipping isotropic and shallow anisotropic contrasts or more than one source of anisotropy at shallow depth. Our calculated orientations for the crust are therefore interpreted as reflecting either the average or the interplay of several sources of azimuthally-varying signals depending of their strength. However, comparing information between stations allows us to draw the same conclusions regionally: in both southern Africa and Madagascar our measurements reflect the interplay between local, inherited structural heterogeneities and crustal seismic anisotropy generated by the current extensional stress field imposed by the southward propagation of the East-African Rift System. A final comparison of our crustal orientations with SKS orientations attributed to mantle deformation further probes the interplay of crustal and mantle anisotropy on SKS measurements.

Description: Summary During the Cenozoic, the Earth experienced multiple first-order geologic events that are likely mantle flow related. These include the termination of large-scale marine inundation in North America in the Paleocene, the late Tertiary rise of Africa relative to other continents, and the long wavelength tilting of Australia since the late Cretaceous, which occurred when the continent approached the South East Asia subduction systems on its northward passage from Antartica. Here we explore a suite of eight high resolution, compressible, global mantle flow retrodictions going back 50 million years ago, using an adoint method with ≈670 million finite elements. These retrodictions show for the first time that these events emerge jointly as part of global Cenozoic mantle flow histories. Our retrodictions involve the dynamic effects from an upper mantle low viscosity zone (LVZ), assimilate a past plate motion model for the tangential surface velocity field, probe the influence of two different present-day mantle state estimates derived from seismic tomography, and acknowledge the rheological uncertainties of dynamic Earth models by taking in four different realizations for the radial mantle viscosity profile, two of which were published previously. We find the retrodicted mantle flow histories are sensitive to the present-day mantle state estimate and the rheological properties of the Earth model, meaning that this input information is testable with inferences gleaned from the geologic record. For a deep mantle viscosity of 1.7 × 1022 Pa.s and a purely thermal interpretation of seismic structure, lower mantle flow velocities exceed 7 cm/yr in some regions, meaning they are difficult to reconcile with the existence of a hotspot reference frame. Conversely, a deep mantle viscosity of 1023 Pa.s yields modest flow velocities (

Description: Intraplate basaltic volcanism commonly exhibits wide compositional ranges from silica-undersaturated alkaline basalts to silica-saturated tholeiitic basalts. Possible mechanisms for the compositional transition involve variable degrees of partial melting of a same source, decompression melting at different mantle depths (so-called “lid effect”), and melt-peridotite interaction. To discriminate between these mechanisms, here we investigated major-trace elemental and Sr-Nd-Mg-Zn isotopic compositions of a suite of intraplate alkaline and tholeiitic basalts from the Datong volcanic field in eastern China. Specifically, we employed Mg and Zn isotope systematics to assess whether the silica-undersaturated melts originated from a carbonated mantle source. The alkaline basalts have young HIMU-like Sr and Nd isotopic compositions, lower δ26Mg (-0.42‰ to -0.38‰) and higher δ66Zn (0.40‰ to 0.46‰) values relative to the mantle. These characteristics were attributable to an asthenospheric mantle source hybridized by carbonated melts derived from the stagnant Pacific slab in the mantle transition zone. From alkaline to tholeiitic basalts, δ26Mg gradually increases from -0.42‰ to -0.28‰ and δ66Zn decreases from 0.46‰ to 0.28‰ with decreasing alkalinity and incompatible trace element abundances (e.g. Rb, Nb, Th and Zr). The Mg and Zn isotopic variations are significantly beyond the magnitude (

Description: Summary The rate of earthquakes with magnitudes Mw ≤ 7.5 in the Ometepec segment of the Mexican subduction zone is relatively high as compared to the neighboring regions of Oaxaca and Guerrero. Although the reason is not well understood, it has been reported that these earthquakes give rise to a large number of aftershocks. Our study of the aftershock sequence of the 2012 Mw7.4 Ometepec thrust earthquake suggests that it is most likely due to two dominant factors: (1) The presence of an anomalously high quantity of over-pressured fluids near the plate interface, and (2) the roughness of the plate interface. More than 5,400 aftershocks were manually detected during the first ten days following the 2012 earthquake. Locations were obtained for 2,419 events (with duration magnitudes Md ≥ 1.5). This is clearly an unusually high number of aftershocks for an earthquake of this magnitude. Furthermore, we generated a more complete catalog, using an unsupervised fingerprint technique, to detect more smaller events (15,593 within one month following the mainshock). For this catalog, a high b-value of 1.50 ± 0.10 suggests the presence of fluid release during the aftershock sequence. A low p-value (0.37 ± 0.12) of the Omori law reveals a slow decaying aftershock sequence. The temporal-distribution of aftershocks shows peaks of activity with two dominant periods of 12h and 24h that correlate with the Earth tides. To explain these observations, we suggest that the 2012 aftershock sequence is associated with the presence of over-pressured fluids and/or a heterogeneous and irregular plate interface related to the subduction of the neighboring seamounts. High fluid content has independently been inferred by magneto-telluric surveys and deduced from heat flow measurements in the region. The presence of fluids in the region has also been proposed to explain the occurrence of slow slip events, low frequency earthquakes, and tectonic tremors.

Description: Summary The increasing density of geodetic measurements makes it possible to map surface strain rate in many zones of active tectonics with unprecedented spatial resolution. Here we show that the strain tensor rate calculated from GPS in the India-Asia collision zone represents well the strain released in earthquakes. This means that geodetic data in the India-Asia collision zone region can be extrapolated back in time to estimate strain buildup on active faults, or the kinematics of continental deformation. We infer that the geodetic strain rates can be assumed stationary through time on the timescale needed to build up the elastic strain released by larger earthquakes, and that they can be used to estimate the probability of triggering earthquakes. We show that the background seismicity rate correlates with the geodetic strain rate. A good fit is obtained assuming a linear relationship ($dot{N} = lambda cdot dot{epsilon }$ where $dot{N}$ is the density of the rate of Mw ≥ 4 earthquakes, $dot{epsilon }$ is strain rate and λ = 2.5 ± 0.1 × 10−3 m−2), as would be expected from a standard Coulomb failure model. However, the fit is significantly better for a non-linear relationship ($dot{N} = gamma _1 cdot dot{epsilon }^{gamma _2}$ with γ1 = 2.5 ± 0.6 m−2 and γ2 = 1.42 ± 0.15). The b-value of the Gutenberg-Richter law, which characterize the magnitude-frequency distribution, is found to be insensitive to the strain rate. In the case of a linear correlation between seismicity and strain rate, the maximum magnitude earthquake, derived from the moment conservation principle, is expected to be independent of the strain rate. By contrast, the non-linear case implies that the maximum magnitude earthquake would be larger in zones of low strain rate. We show that within areas of constant strain rate, earthquakes above Mw4 follow a Poisson distribution in time and and are uniformly distributed in space. These findings provide a framework to estimate the probability of occurrence and magnitude of earthquakes as a function of the geodetic strain rate. We describe how the seismicity models derived from this approach can be used as an input for probabilistic seismic hazard analysis. This method is easy to automatically update, and can be applied in a consistent manner to any continental zone of active tectonics with sufficient geodetic coverage.

Description: SUMMARY The Kohistan Island Arc (KIA) occupies the northwestern region of the Himalayan Mountains, sandwiched between Asia and India plates. Its formation, collision with plate boundaries, and evolution has been controversially discussed for a couple of decades. To better understand this, a palaeomagnetic study has been conducted on the Jutal dykes (ca. 75 Ma), intruded in the northeastern part of the KIA. Comprehensive rock magnetic investigations reveal that the magnetic carrier minerals are pyrrhotite and magnetite. An intermediate temperature component (ITC) predominates the natural remanent magnetization and shows good coincidence within-site; it is carried by pyrrhotite and is considered reliable, yielding a mean direction at Dg/Ig = 11.5°/39.9° (kg = 28.4, α95 = 3.5°) before and Ds/Is = 8.6°/12.1° (ks = 5.1, α95 = 9.1°) after tilt correction. A high-temperature component that is carried by magnetite exhibits random distribution within-site. The fold test for the ITC is negative, indicating a post-folding origin. Scanning electron microscopy combined with energy-dispersive X-ray spectroscopy indicates that the magnetic carrier minerals were influenced by metamorphism or thermochemical fluids. The comparison of mean palaeolatitude (22.6 ± 3.5°N) of the ITC with the collisional settings and thermal history of the study area implies that the remagnetization occurred at ∼50–35 Ma, consistent with the previous reported palaeomagnetic data of the KIA. We propose a tectonic model that shows the evolution of the Jutal dykes, supporting the concept that India collided with the KIA first, followed by a later collision with Asia.