Tidal Changes Research Articles

Tides of Change—Barriers and Facilitators to Beach Accessibility for Older People and People with Disability: An Australian Community Survey

The beach is Australia's most popular recreational destination with participation in beach-based activities associated with a wide range of health and wellbeing benefits. Unfortunately, access to beach environments is not possible for many older people and people with a disability. The purpose of this study was to investigate the barriers and facilitators of beach accessibility using a framework that recognises the complex interconnections between blue space, accessibility, physical activity, and health and wellbeing. A 39-item anonymous online cross-sectional survey was developed and administered to explore the perspectives of older people and people with a disability regarding beach accessibility. In total, 350 people completed the survey (69% female, age range 2-90 years (mean = 52)). Disability was reported by 88% of respondents, with 77% requiring a community mobility aid. Two-thirds (68%) of respondents were unable to visit the beach as often as they wanted, with 45% unable to visit at all. The most frequently reported barriers to beach access included difficulty moving on soft sand (87%), no specialised mobility equipment (75%), and inaccessible lead-up pathways (81%). If beach access was improved, respondents reported they would visit the beach more often (85%), for longer (83%), and have an improved experience (91%). The most frequently reported facilitators to beach access were the presence of accessible lead-up pathways (90%), sand walkways (89%), and parking (87%). Older people and people with disability have limited beach access, primarily due to a lack of accessible equipment, excluding them from the wide range of health benefits associated with visiting the beach.

The Ocean Surface Current in the East China Sea Computed by the Geostationary Ocean Color Imager Satellite

High-frequency observations of surface current field data over large areas and long time series are imperative for comprehending sea-air interaction and ocean dynamics. Nonetheless, neither in situ observations nor polar-orbiting satellites can fulfill the requirements necessary for such observations. In recent years, geostationary satellite data with ultra-high temporal resolution have been increasingly utilized for the computation of surface flow fields. In this paper, the surface flow field in the East China Sea is estimated using maximum cross-correlation, which is the most widely used flow field computation algorithm, based on the total suspended solids (TSS) data acquired from the Geostationary Ocean Color Imager satellite. The inversion results were compared with the modeled tidal current data and the measured tidal elevation data for verification. The results of the verification demonstrated that the mean deviation of the long semiaxis of the tidal ellipse of the inverted M2 tide is 0.0335 m/s, the mean deviation of the short semiaxis is 0.0276 m/s, and the mean deviation of the tilt angle is 6.89°. Moreover, the spatially averaged flow velocity corresponds with the observed pattern of tidal elevation changes, thus showcasing the field’s significant reliability. Afterward, we calculated the sea surface current fields in the East China Sea for the years 2013 to 2019 and created distribution maps for both climatology and seasonality. The resulting current charts provide an intuitive display of the spatial structure and seasonal variations in the East China Sea circulation. Lastly, we performed a diagnostic analysis on the surface TSS variation mechanism in the frontal zone along the Zhejiang coast, utilizing inverted flow data collected on 3 August 2013, which had a high spatial coverage and complete time series. Our analysis revealed that the intraday variation in TSS in the local surface layer was primarily influenced by tide-induced vertical mixing. The research findings of this article not only provide valuable data support for the study of local ocean dynamics but also verify the reliability of short-period surface flow inversion of high-turbidity waters near the coast using geostationary satellites.

Climate Change and the Sea: A Major Disruption in Steady State and the Master Variables.

Since the beginning of the industrial revolution, humans have burned enormous quantities of coal, oil, and natural gas, rivaling nature's elemental cycles of C, N, and S. The result has been a disruption in a steady state of CO2 and other greenhouse gases in the atmosphere, a warming of the planet, and changes in master variables (temperature, pH, and pε) of the sea affecting critical physical, chemical, and biological reactions. Humans have also produced copious quantities of N and P fertilizers producing widespread coastal hypoxia and low dissolved oxygen conditions, which now threaten even the open ocean. Consequently, our massive alteration of state variables diminishes coral reefs, fisheries, and marine ecosystems, which are the foundation of life on Earth. We point to a myriad of actions and alternatives which will help to stem the tide of climate change and its effects on the sea while, at the same time, creating a more sustainable future for humans and ecosystems alike.

Tide-induced infiltration and resuspension of microplastics in shorelines: Insights from tidal tank experiments

In the present study, the infiltration and resuspension of microplastics (MPs) in a slope substrate under the influence of repeated tidal forces were investigated using a tidal tank. In the scenario in which MPs were placed on the top of the slope, increasing numbers of particles were observed on the water surface with the increase in tidal cycles. More particles of smaller equivalent particle diameter (dMP) and low density floated to the water surface. The horizontal positions (positive toward the lower tide zone) of MPs showed significant positive correlation with the shortest length c of MPs, MP density, MP weight, dMP, and Corey shape factor, whereas they showed significant negative correlation with the rate of tidal level change and the longest length a of MPs. The vertical positions (positive in the downward direction) of MPs showed significant positive correlation with the shortest length c of MPs, MP density, MP weight, dMP, and Corey shape factor, while they demonstrated significant negative correlation with the largest cross-section area and surface tension of MPs. In the scenario in which MPs were placed at the bottom of the tank, the smaller and low-density particles had a higher possibility of moving upward to the water surface under repeated tidal forces. High-density particles also migrated to the water surface due to the surface tension force. Further, a lower rate of tidal level change contributed to more floating of particles. The horizontal positions of MPs showed significant positive correlation with MP density, while they demonstrated significant negative correlation with the largest cross-section area and surface tension of MPs. The vertical positions of MPs showed significant positive correlation with the longest length a of MPs, MP density, MP weight, and dMP. These results imply that large, high-density, and less flatty particles tend to be distributed in the lower tidal zone and deeper substrate layers. These findings can help understand the redistribution of MPs and assess their risk in the shoreline environment.

Spatio-Temporal Changes of Mangrove-Covered Tidal Flats over 35 Years Using Satellite Remote Sensing Imageries: A Case Study of Beibu Gulf, China

Tidal flats provide ecosystem services to billions of people worldwide; however, their changing status is largely unknown. Several challenges in the fine extraction of tidal flats using remote sensing techniques, including tide-level and water-edge line changes, exist at present, especially regarding the spatial and temporal distribution of mangroves. This study proposed a tidal flats extraction method using a combination of threshold segmentation and tidal-level correction, considering the influence of mangrove changes. We extracted the spatial distribution of tidal flats in Beibu Gulf, Southwest China, from 1987 to 2021 using time-series Landsat and Sentinel-2 images, and further analyzed the dynamic variation characteristics of the total tidal flats, each coastal segment, and the range of erosion and silting. To quantitatively investigate the interaction between tidal flats and mangroves, this study established a regression model based on multi-temporal tidal flats and mangrove data. The results indicated that the overall accuracy of the tidal flat extraction results was 93.9%, and the kappa coefficient was 0.82. The total area of tidal flats in Beibu Gulf decreased by 130 km2 from 1987 to 2021, with an average annual change of −3.7 km2/a. In addition, a negative correlation between the tidal flat change area and mangrove change area in Shankou, Maowei Sea, and Pearl Bay was observed, with correlation coefficients of −0.28, −0.30 and −0.64, respectively. These results demonstrate that the distribution of tidal flats provides a good environment and expansion space for the rapid growth of mangroves. These results can provide references for tidal flats’ resource conservation, ecological health assessment, and vegetation changes in coastal wetlands in China and other countries in Southeast Asia.

Seasonal Assessment of Ecological and Human Health Risks of Trace Metals in the Saigon River Surface Water, Vietnam

AbstractRiver water pollution with potentially toxic elements can be quantified via hydrological studies, while its potential impacts on the ecosystem and human health can be assessed via the indices of ecological risk (RI) and human health hazard (HI). This study aims to assess potential ecological and human health risks of trace‐element pollution of the Saigon River surface water, based on seasonal, position, and tidal changes. Surface water samples are taken during dry and rainy seasons and analyzed for Cu, Cr, Mn, Pb, Ni, and Zn. The trace‐element concentrations are used to compute RI and HI. Of the six elements, Cu accounted for 87.7 and 12.5% of the total variance of RI and HI, respectively. The RI and HI values are strongly correlated and are higher in dry season than in rainy season. The HI value of >1 for children suggests that children's health may be sensitive to the adverse impacts of the current status of the trace‐element pollution. Overall, the trace‐element pollution of the river surface water is at a moderate ecological risk in dry season but at a low ecological risk in rainy season. The potentially hazardous elements can have detrimental impacts on children's health in both seasons.

Total Maximum Daily Load Application Using Biological Oxygen Demand, Chemical Oxygen Demand, and Ammoniacal Nitrogen: A Case Study for Water Quality Assessment in the Perai River Basin, Malaysia

Water shortage has been an issue for urbanized areas. For the Penang state in Malaysia, it is forecast that there will be a significant increase in water demand in the future. Penang authorities in Malaysia are trying to find an alternative water source to overcome the problem, with one of the options being the Perai River catchment. However, the river water quality was found to be polluted and not suitable to be used for water extraction for domestic consumption. This paper aims to study the pollution level variation due to changes in rainfall during the year in the Perai River Basin, and estimate the TMDL of the river in a particular case for BOD, COD, and NH3N parameters. A water quality model was developed for the Perai River, Jarak River and Kulim River using InfoWorks ICM. The year 2016 was selected as a model event due to data availability. BOD, COD and NH3N concentrations were used for TMDL calculation, and the load duration curve approach was used to estimate TMDL. The tidal effect at the downstream of the Perai River was found to impact the data analysis in the river stretch. It was found that pollutant load exceedance was the highest during the rainy season and the problematic pollutant was NH3N. Thus, local authorities need to focus on tidal and seasonal change factors when developing action plans to manage water quality issues in this basin.

Wave–Tide Interaction for a Strongly Modulated Wave Field

Abstract Observations from Coastal Data Information Program (CDIP) moored buoys off the coast of Florida reveal tidally driven wave–current interactions that modify significant wave heights by up to 25% and shift peak periods by up to a second. A case study at Fernandina Beach, Florida, shows surface waves steepening on following tidal currents and becoming less steep on opposing tidal currents, with the largest modulations occurring in the long-period swell band. To better understand tidal modulations as a function of the phase of the tide, we use simplified analytical and numerical solutions to the equations of geometrical optics and conservation of wave action under the assumption of a one-dimensional tide acting as a progressive shallow-water wave. The theoretical frameworks allow us to identify parameters that characterize the magnitude of variation in surface waves due to tidally induced currents and changes in water depth. We compute modulations to the omnidirectional and directional wave spectrum (between 0.05 and 0.15 Hz), as well as characteristic bulk parameters such as significant wave height and peak period. The theory is corroborated using directional wave and surface current observations from the Fernandina Beach CDIP station (located in water of average depth of 16 m). We find that the numerical results reproduce the observed wave modulations due to tidal currents and changes in water depth. Specifically, surface waves traveling in the direction of the tide are strongly modulated, and the relative speeds between the tide and surface waves set the sign and magnitude of these modulations. Given knowledge of tidal currents, water-depth variations, and wave climatology, theoretical and numerical predictions may be used to provide both statistical and instantaneous estimates of wave-height variations due to tides. Because operational forecasts and nowcasts do not routinely include tides or currents, these findings can help to accurately represent nearshore surface wave variability.

Influence of Complex Hydraulic Environments on the Mechanical Properties of Pile-Soil Composite Foundation in the Coastal Soft Soil Area of Zhuhai

Based on a plain concrete pile composite foundation project in the coastal area of Zhuhai, considering the complex hydraulic load environment induced by tidal water-level changes, finite element simulations and parameter calibrations were carried out to determine the physical and mechanical properties of plain concrete pile composite foundation. The hardening soil small (HSS) model, which can be used to simulate the complex mechanical behavior of soft soil under small strain, was selected for modeling analysis. Model parameters were calibrated through resonance column tests, triaxial consolidation drainage loading and unloading shear tests. The complex hydraulic loads were analyzed, including the effects of cyclic tidal action and the sudden rise and fall of the water level induced by strong storm surges on the force, deformation of plain concrete piles, and the mechanical seepage properties of soft soil around piles. The results indicate that: (1) Compared with coastal soft soil in Shanghai, Zhoushan, Tianjin, and other areas, the soft soil in the Zhuhai area has a smaller dynamic shear modulus, cohesion and internal friction angle, and worse engineering properties. (2) The sudden rise of water level leads to a sudden change in the pore pressure of the groundwater, which induces a large deformation of the pile-soil composite foundation. If the foundation on the offshore (dike) side exhibits the most prominent deformation and foundation damage, such as uneven settlement is prone to occur. (3) The offshore side pile is most affected by the hydraulic loads. The deformation of the pile body along the pile body is uneven and the deformation of the upper pile body is relatively large, which may cause fracture damage.

Anthropogenic interventions on land neutrality in a critically vulnerable estuarine island ecosystem: a case of Munro Island (India)

All landscapes, including estuarine islands, normally try to restore their geomorphic isostasy in all anthropogenic interventions on land dynamics. Munroe Island has been experiencing drastic environmental degradation, such as land subsidence, severe tidal/monsoon flooding, subsidence of build-ups and a drastic decay in agricultural productivity. This paper examines the role of anthropo-geomorphic interventions causing for the land degradation in Munroe Island through a multidisciplinary approach. Multidated, multiresolution satellite products and published maps, spanning a period of about six decades from 1960 to 2021, were used to understand the different geomorphic and geographical processes in the study area. Evaluation of the temporal bathymetric datasets, salinity measurements of the river and estuary, borehole data logs of the area and electrical resistivity surveys of the island were analyzed to find out the causative factors for the disturbances in the land neutrality, along with the tidal hydrodynamic changes in the region. The study shows about 14% of the total land area was vanished during the study period, and more than 25% of the area is under stress, leading to further land degradation. More than 500 households are forced to vacate their residence due to land subsidence/flooding. Lack of required freshwater and sediment supply from the Kallada river after the construction of the Thenmala reservoir in the Kallada river as well as the uncontrolled sand mining prevailed are the key factors for the environmental degradation of Munroe Island. The paper describes the role and colinkages of human-induced hydrogeomorphic interventions on a geomorphic system, in charge of the environmental degradation and land subsidence crisis of an estuarine island ecosystem and discusses the concerns related to the management strategies of such region.

Mapping seagrasses on the basis of Sentinel-2 images under tidal change

Tidal variations make the water bodies in satellite remote sensing images on different shooting dates have different inundation ranges and depths. Although the underwater substrates do not change, the spectral properties differ due to attenuation effects. These differences have an impact on the results when multi-temporal remote sensing images are used to analyze seagrasses. This paper proposes a remote sensing mapping method for seagrasses taking the tidal influence, using the seagrasses growth area in Xincun Bay, Hainan Province, China as a case study. a) The seagrasses growth area was determined from remote sensing images. The seagrasses were divided into two types: the seagrasses exposed to water surface or tidal flats (non-submerged seagrasses) and the seagrasses submerged in water (submerged seagrasses). b) The spectral features of seagrasses in Sentienl-2 image were analyzed. We found that the spectral characteristics of non-submerged seagrasses were similar to terrestrial vegetation and these seagrasses could be extracted by using NDVI. The submerged seagrasses spectral was different, forming a reflection peak at the first vegetation red edge band (i.e.705 nm) in Sentinel-2 images. This reflection peak was used to design the Submerged Seagrasses Identification Index (SSII) for extracting underwater seagrass. c) The extraction results of non-submerged seagrasses and submerged seagrasses were merged to map the seagrasses in the study area. The experimental results show that the mapping method proposed in this study can fully consider the influence of tidal changes in remote sensing images on seagrasses identification. The SSII constructed based on Sentinel-2 images extracted submerged seagrasses effectively. This study will provide references to remote sensing mapping of seagrasses and integrated ecological management in coastal zones.

Rapid urbanization and global warming significantly impact tidal dynamics in the Pearl River Estuary, China

Due to the dual impacts of anthropogenic disturbances and climate change, tidal hydrodynamics of estuaries have been intensively investigated for their potential to increase flood risk and devastate coastal habitats and human communities. Climate change leads to rises in sea levels, which can amplify regional tides and increase tidal ranges. This study takes the estuarine area of the Pearl River (PRE), one of the fastest-developed estuaries in China, as an example to explore the effects of land cover changes and sea level rise on tidal dynamics. Annual mean tidal levels were collected at 26 major hydrological stations from five major rivers/waterways. To better understand the temporal changes in tidal levels, we analyzed the trends and periodic patterns through linear regression and Complex Morlet wavelet analyses. For the main Pearl River outlet, where conversion of agricultural land to urban use had been primarily taking place, the tidal level changes were significantly affected by increased built-up areas and decreased cultivated land areas. On the other hand, in the middle to upper reaches of the Xi River and Shunde-Hengmen Waterway, we observed decreased tidal levels with decreases in the agricultural lands. No significant correlations were found between annual mean tidal levels and land cover changes downstream of the main rivers. Instead, the tidal level changes at locations close to the coastline were more related to sea level rise, with an average of 0.002 m per year increase. This study highlighted the importance of human activity and sea level rise impacts on tidal dynamics. A systematic understanding of the tidal changes in the estuary is expected to help enhance mechanisms of estuarine hydrology, which could serve as the basis for improving the management and practice of coastal regions in coping with climate change and estuary protection.

The effect of tidal range and mean sea-level changes on coastal flood hazards at Lakes Entrance, south-east Australia

Despite being well-documented in other countries, the roles that anthropogenically induced changes and natural variability in tidal processes play in modulating coastal flood frequencies have not been investigated in Australia. Here we conduct a brief assessment of changes in tidal variability around Australia. We then apply a simple attribution framework to quantify the separate and joint effects of tidal range changes and increasing relative mean sea level on nuisance flood frequency at the location with the largest relative changes in tidal range, Lakes Entrance, Victoria. To understand how these changes in variability affect flood hazards, we consider a nuisance flood threshold based on recent coastal flood impact surveys. Results show that increases in the heights of high tides over recent years have exerted a large influence on coastal flood frequencies. These recent changes are potentially linked to changes in channel dredging regimes. We show that 93% of nuisance flood days since 2009 would not have occurred without these tidal range changes or the coincident increases in the mean sea level. We demonstrate the importance of considering tidal processes in estuarine coastal flood hazard assessments for future planning, even if these processes do not represent a substantial flood threat today. We discuss the implications of this study for future work on estuarine flood hazards and the benefits of considering impact-based thresholds in the assessment of such hazards.

Not Another Plant-Based Documentary: A Critical Review of Eating Our Way to Extinction

Despite mounting evidence that industrial animal agriculture is a formidable force of climate change and mass extinction, many humans remain impervious to this knowledge. Eating Our Way to Extinction is a timely documentary that takes this issue head on. This film review is guided by Alexandra Juhasz’s explanation of media praxis as ‘an enduring, mutual, and building tradition that theorizes and creates the necessary conditions for media to play an integral role in cultural and individual transformation’ (299). Eating Our Way to Extinction attends to some of the most popular strawman arguments against veganism and is widely accessible. That being said, it falls short of its sociopolitical potential because it is beholden to the capitalist-colonial norms of self-interested individualism, promotion of consumerism over movement-building, and using Indigenous peoples as a means to an end. Eating Our Way to Extinction contrasts a worldview based on extraction and domination with one that could actually shift the tide of climate change. It then follows the logic of the extractive worldview by promoting self-interested solutions to a problem that is only exacerbated by capitalism. The fact that Eating Our Way to Extinction acknowledges that Indigenous peoples are more adept at living in an ecologically harmonious way, then silos its viewers into the very mindset that is driving the problem is where the documentary falls flat. At its heart, Eating Our Way to Extinction relies on the Western colonial logics of individualism and capitalism that undercut the social justice demands of veganism.

Analysis on the Influence Factors of Heat Dissipation of Undersea Data Center Based on Multiple Linear Regression

Deploying data centers in the waters near coastal cities can greatly reduce the distance between data and users, not only eliminating the need to occupy land resources, but also saving energy consumption, which is a completely green and sustainable data center solution. Under the background of resource conservation and green development, this paper puts forward the project of undersea data center heat dissipation. Through the establishment of multiple linear regression model, the influence of tidal and seasonal changes on the heat dissipation of undersea data center is analyzed. Finally, it is concluded that the seasonal changes have the greatest impact on the heat dissipation of the undersea data center container. In the future, the placement strategy of data center and the methods to improve the heat dissipation efficiency will be further discussed.

Anomalously large seasonal modulations of shallow water tides at Lamu, Kenya

Understanding seasonal tidal changes is vital for accurate tidal prediction which is essential for numerous practical purposes, such as flood warning and navigation. It is common knowledge that main constituents show significant seasonal modulations in the global ocean due to non-astronomical factors such as river discharge, sea ice and ocean stratification. Less known, and less studied, is the seasonal variability of shallow water constituents. In this paper, we revisit the abnormally large semi-annual modulation of shallow water constituents at Lamu indicated by a previous study. We find that large semi-annual modulations of shallow water tides at Lamu are mostly not physical, but artifacts induced by unresolved nearby nonlinear shallow water tides due to short length of records. We also propose a novel method based on the ‘credo of smoothness’ to extract real semi-annual modulations of shallow water tides. Tidal harmonic analysis and other widely-used time series methods are generally frequency-depend, which means that they can only extract the amplitude and phase of one specific tidal frequency, but cannot distinguish different origins of a tidal constituent. It is suggested that care must be taken to ensure the obtained tidal amplitude/phase series are really reflecting tidal changes and are not artifacts of frequency-depend methods.

Desiccation Stress Tolerance in Porphyra and Pyropia Species: A Latitudinal Analysis along the Chilean Coast.

One of the most important factors regulating the distribution and abundance of seaweeds is desiccation, triggered mainly by tidal changes and climatic variation. Porphyra and Pyropia species have evolved multiple strategies to tolerate desiccation stress; however, how these tolerance strategies differ in these species inhabiting different latitudes is still unknown. In this context, we analyzed, in situ, the physiological responses of these species (collected from 18° S to 41° S along the Chilean coast) to desiccation stress using biochemical and molecular analyses. The hyper-arid terrestrial climate of northern Chile, with high evaporation and lack of constant rain determines a very steep increase in desiccation stress in the upper intertidal during low tide for these species. Accordingly, the results showed that, in comparison with the southernmost populations, the Porphyra/Pyropia species from the north zone of Chile (18°-30° S) exhibited higher contents of lipoperoxide and carbonyls (1.6-1.9 fold) together with higher enzymatic activities, including ascorbate peroxidase, catalase, peroxiredoxin, and thioredoxin (2-3-fold). In addition, a substantial expression of cat, prx, and trx transcripts during desiccation was demonstrated, mainly in the northernmost populations. These results provide evidence of (i) significant activation of antioxidant enzymes and transcripts (principally cat and prx); (ii) participation of phenolic antioxidant compounds as a highly plastic physiological strategy to cope with desiccation; and (iii) the activation of the tolerance responses was affected by species latitudinal distribution. Thus, for the first time, this study integrated the biochemical and genetic responses of diverse Porphyra/Pyropia species to better understand their physiological dynamics of tolerance over a wide latitudinal range.

Improving tidal turbine array performance through the optimisation of layout and yaw angles

Tidal stream currents change in magnitude and direction during flood and ebb tides. Setting the most appropriate yaw angles for a tidal turbine is not only important to account for the performance of a single turbine, but can also be significant for the interactions between the turbines within an array. In this paper, a partial differentiation equation (PDE) constrained optimisation approach is established based on the Thetis coastal ocean modelling framework. The PDE constraint takes the form here of the two-dimensional, depth-averaged shallow water equations which are used to simulate tidal elevations and currents in the presence of tidal stream turbine arrays. The Sequential Least Squares Programming (SLSQP) algorithm is applied with a gradient obtained via the adjoint method in order to perform array design optimisation. An idealised rectangular channel test case is studied to demonstrate this optimisation framework. Located in the centre of the computational domain, arrays comprised of 12 turbines are tested in aligned and staggered layouts. The setups are initially optimised based on their yaw angles alone. In turn, turbine coordinates and yaw angles are also optimized simultaneously. Results indicate that for an aligned turbine array case under steady state conditions, the energy output can be increased by approximately 80\% when considering yaw angle optimisation alone. For the staggered turbine array, the increase is approximately 30\%. The yaw optimised staggered array is able to outperform the yaw optimised aligned array by approximately 8\%. If both layout and the yaw angles of the turbines are considered within the optimisation then the increase is more significant compared with optimising yaw angle alone.

Statistical study on the regional characteristics of seismic activity in and around Japan: frequency-magnitude distribution and tidal correlation

AbstractWe propose a statistical analysis method to identify common features of seismic activity that are indistinguishable from most other seismicity, and to find anomalous activity that differs from these common features. Using the hypocenter catalog of earthquakes which occurs in and around Japan during the past 20 years, we apply this method for the parameters of the frequency-magnitude distribution and a parameter that expresses the correlation of seismicity with tides as indices, with a focus on objectively understanding the regional characteristics of seismicity. As a result, we extracted a "typical" probability density distribution of each index value common to the most analysis regions and "anomalous" regions with index-value distributions that differ significantly from the typical distributions. In terms of the frequency-magnitude distribution, most estimated values of indices in the anomalous activity areas can be explained as variations corresponding to the effects of fluids, interplate coupling, and stress fields that control faulting styles that have been pointed out in previous studies. By extracting typical index values for the frequency-magnitude distribution, common features of the frequency-magnitude distribution that depend on the earthquake occurrence interval were identified. Although seismicity showed no clear correlation with tides, the index value for tidal correlation changes to reflect the proportion of earthquakes occurring in a series of periods shorter than the tidal period; it is therefore useful as an index to capture the characteristics of such earthquake occurrence intervals. We also show that the typical probability density distribution of these index values can be represented by existing models or their extensions. By using the proposed models as a reference, it is possible to quantify the degree of anomaly using the same concept as that of the method presented here; hence, such a method should be applicable to monitor seismic activity. Graphical Abstract

Signature of Tidal Sea Level in Geomagnetic Field Variations at Island Lampedusa (Italy) Observatory

In this work, we analyze the geomagnetic field measurements collected from 2017 to 2020 at the Italian observatories of Lampedusa and Duronia (an island and inland site, respectively) for investigating a possible signature of the tidal sea water level changes on the local magnetic variations. We obtain the following main results: (a) evidence of the geomagnetic power spectral peaks at the solar and lunar tidal main frequencies at both sites is found; (b) by using a robust fit procedure, we find that the geomagnetic field variations at Lampedusa are strongly influenced by the lunar tidal variations in the sea level, while at Duronia, the main effects on the geomagnetic field variations are associated with diurnal solar ionospheric tides; (c) a single-station induction arrows (SSIAs) investigation reveals different behaviors between Lampedusa and Duronia. Specifically, Lampedusa shows that the induction arrows in different frequency ranges point toward different directions with different amplitudes, probably related to the surrounding regions with different water depths, while Duronia shows a persistent coast effect, with the induction arrows pointing toward the Adriatic sea; and (d) a Superposed Epoch Analysis reveals, only for Lampedusa, a close relationship between SSIAs with a frequency of >2 mHz (<1.3 mHz) and the sea level variations driven by the astronomical O1 tide, indicating an amplitude intensification of ∼4×10−3 (∼5×10−3) and an azimuthal angle increment of ∼3∘ ( ∼9∘), in correspondence to a 1 cm sea level increase.