Monitoring the physical forcing of resuspension events in the Thermaikos Gulf—NW Aegean during 2001–2003

This work constitutes an attempt to assess the relative importance of natural versus anthropogenic forcing for sediment resuspension on the shelf of the Thermaikos Gulf (NW Aegean) through a 2-year experiment in the framework of the E.U. project INTERPOL. Four periods of different hydrographic characteristics were identified, including two periods of stronger vertical homogenisation and two of stratified conditions. The former periods were characterized by stronger near-bed currents, while relatively weak internal wave motions dominated the periods of stratification. The near-bed currents showed strong coherence with the wind during the period of full homogenisation, whereas during stratification periods the wind provided indirect forcing evident mostly in the internal-wave bands. The site was too deep for the surface waves to cause any sediment resuspension. In conclusion, the observed near-bed currents at the site of interest did not appear to produce significant local resuspension of sediment; notably, the current-turbidity correlation suggested a shoreward transport of suspended material. The large-scale turbidity variability was rather related to the onset of the trawling period in mid-October 2001 and the increased riverine discharge in late summer/autumn 2002.     

Temporal and spatial variations in the geochemistry of major and minor particulate and selected dissolved elements of Thermaikos Gulf, Northwestern Aegean Sea

The objectives of this study were to examine both spatial and temporal changes of particulate major elements and minor metals, as well as dissolved Mn and Cd, in the waters of Thermaikos Gulf. Collections of water and suspended particulate matter (SPM), as depth profiles (5–8 depths), were undertaken at 10 principal stations, essentially on a N–S traverse of the western side of the Gulf.One of the principal aims of the study was to observe if there was any change in the patterning of the elements between the three occupations of the stations: (a) in September 2001, immediately before the commencement of trawling; (b) in October 2001, whilst fishing was active; and (c) in winter/early spring conditions (February 2002), when fishing was still active, but after a change of river/atmospheric conditions.Bottom (20 m) waters were dominated by sediment resuspension; this was identified by concentration changes in the aluminosilicate elements (e.g. Al, Ti, K, Fe) of the SPM. A two- to three-fold increase occurred between September and October, caused probably by trawling; this was sustained at the offshore stations, in February. During February, the western inshore stations showed little sediment resuspension, caused by extreme winter cooling and the sinking of water. Consequently, a N–S density discontinuity existed at all water depths, which prevented the thermohaline cyclonic circulation from penetrating into the western seaboard of the Gulf. The distribution of dissolved and particulate Mn in the lower waters was due to redox cycling of the element at the benthic boundary; this was more intense in the north, where the organic supply was higher.Biogenic element concentrations and Ca/Al, Si/Al ratios showed no evidence that trawling activity promoted higher biological production. Strong correlations of Co, Cr, Ni and V, with Al and K, showed that these elements were associated strongly with detrital aluminosilicates. However, the variable association of Cd, Pb and Zn, with K (and Al), especially in the upper waters, implied an anthropogenic source derived from the rivers and the city of Thessaloniki. Examination of the Kd's of Cd showed a two-order of magnitude decrease with depth, caused by resuspension and possible advection of relatively unpolluted sediments, into the western Gulf.     

Th:U disequilibrium as an indicator of sediment resuspension in Thermaikos Gulf, northwestern Aegean Sea

²³⁴Th:²³⁸U disequilibria have been used extensively as tracers of particle dynamics in marine environments. ²³⁴Th (t_1/2=24.1 days) can be used as “proxy” for particle reactive pollutants, due to their similar rapid rate of scavenging onto particles and subsequent removal from the water column, to the sediments. Radioactive disequilibrium can be exploited to determine the rates and time-scales of processes occurring over days to months; in this instance the residence times of dissolved and particulate species with the benthic nepheloid layer (BNL).Three sampling cruises were undertaken in Thermaikos Gulf (NW Aegean Sea) during contrasting periods, to examine the impact of natural and anthropogenic activity on sediment resuspension. September and October 2001 represented background and trawling periods, respectively; January 2002 represented a mixed period, of trawling and storms.Dissolved ²³⁴Th is scavenged actively at the BNL, in the presence of suspended particulate material (SPM), with a mean residence time of 16 days. There is a weak inverse correlation between dissolved ²³⁴Th residence time and SPM concentration in the BNL, with the shortest residence times occurring during October 2001. No relationship was observed between particulate ²³⁴Th activities and SPM concentrations, indicating that particles are rapidly removed from the system, either by sinking or advection. The mean particulate ²³⁴Th residence time is 5 days.     

Dense water formation and cascading in the Gulf of Thermaikos (North Aegean), from observations and modelling

Observations of dense water formation on the shelf of the Gulf of Thermaikos (North Aegean) are presented, based mainly on continuous monitoring of temperature and currents, during the winter of 2001–2002, at an instrumented mooring and a CTD survey carried out in early February 2002. A 2.5-month realistic simulation, corresponding to the period of observation, was performed to investigate the processes of dense water formation and cascading. The simulation is first compared to the main characteristics of the dense water, time variation of bottom temperature and spatial distribution of the dense water on the shelf. Subsequently, the simulation is used (a) to show that the formation of dense water takes place within the semi-enclosed Thessaloniki Bay and (b) to explain the intermittence of cascading out of the bay in relation to wind variations. The pathways of the dense water through the shelf are investigated with an emphasis on the role of the bottom slope and friction in the Ekman layer. The export of dense water towards the open sea occurs primarily along the slope bounding the western coast.     

Dinoflagellate cysts in recent marine sediments from Thermaikos Gulf, Greece: Effects of resuspension events on vertical cyst distribution

A qualitative and semi-quantitative study of recent dinoflagellate cysts has been undertaken in the NW part of Aegean Sea, Thermaikos Gulf (Eastern Mediteranean), before (September 2001), during (October 2001) and after 120 days (February 2002) of intensive trawling activities. This is the first survey of recent dinoflagellate cysts from Greek marine coastal environments. Sediment samples were collected with a corer and the vertical distribution of the cysts was studied at five different layers, from 0 to 10 cm. Dinoflagellate cysts were both abundant and diverse. Cysts were found over the whole sampling area and periods, with concentrations ranging between 247–3202 cysts cm⁻³. Thirty-six cyst types were encountered, of which 32 were identified to species level, representing 12 genera. It seems that significant local resuspension, related to the onset of the trawling period and stirring up of the sediment, contributed to mixing of the upper layers, resulting to more homogenous cyst profiles in the sediment. Viable cysts constituted 16–60% of the total cyst abundance. The abundance peaks of viable cysts within the subsurface sediment layers, observed during the undisturbed period, disappeared during October. In February, the reduction of cyst concentration was associated to a loss of viable cysts, whilst the ratio of viable/empty cysts ranged between 0.30 and 0.67. The abundance of the different dinoflagellate species, in their active form, was monitored in order to detect any relationship between the concentration of cysts in the top 10 cm of sediment and blooms of algae in the water column. Cysts of potentially toxic species, causing Paralitic Shellfish Poisoning (PSP), such as Alexandrium cf. tamarense, A. cf. affine, A. cf. minutum, as well as Gymnodinium catenatum, were detected in the cyst survey.     

Comparison of particulate matter distribution, in relation to hydrography, in the mesotrophic Skagerrak and the oligotrophic northeastern Aegean Sea

The seasonal vertical distribution of particulate matter (PM) was studied in two contrasting areas: (a) the mesotrophic Skagerrak (in the North Sea); and (b) the oligotrophic northeastern Aegean Sea (eastern Mediterranean). Similarities and differences of the PM distribution in the two areas are assessed with respect to the prevailing hydrographic conditions and the PM composition. Hydrographic conditions in both of the areas are characterised by strong density gradients, resulting from the inflow of low-salinity water, i.e. Baltic Sea water and Black Sea water for the Skagerrak and the northeastern Aegean Sea, respectively.Enhanced primary production and particles mainly of biogenic origin characterise the mesotrophic Skagerrak, whereas five-fold lower particle concentrations appeared in the oligotrophic Aegean Sea. The input of riverine particulates was limited in both of the areas. In the Skagerrak, the strong stratification resulted in particle accumulation on/above the pycnocline and the development of pronounced intermediate nepheloid layers (INLs). The pycnocline-related INLs were formed entirely by dinoflagellates. The pycnocline hindered the vertical movement and sinking rates of particles, thus favouring primary production. Particle horizontal advection along the density discontinuities was probably enhanced. This pattern was not observed in the stratified waters of the northeastern Aegean Sea, probably due to the very low particle concentrations and/or the fact that phytoplankton maxima appeared in deeper waters. Pronounced INLs were identified in the Skagerrak below the pycnocline; these are attributed to accumulated or advected dinoflagellate skeletal remains mixed with clay mineral particles. This was revealed only by means of SEM observations. X-ray diffraction analysis could not provide information on the type of phytoplankton present, because dinoflagellates form their skeletons from organic material. Frontal stations in the northeastern Aegean Sea exhibited pronounced vertical movement of particles towards the deeper waters. Benthic nepheloid layers (BNL) were observed in the Skagerrak; these were related to the resuspended fine-grained surface sediments. In the northeastern Aegean Sea, although near-bottom current velocities were sufficient to resuspend surface sediments, resuspension occurred only episodically. The BNLs here are related mostly to near-bottom phytoplankton growth.     

Chemical composition of short sediment cores from Thermaikos Gulf (Eastern Mediterranean): Sediment accumulation rates, trawling and winnowing effects

Four cores recovered within the framework of the INTERPOL Project have been analysed for their grain size and geochemistry; sediment accumulation rates (SARs) were also determined from ²¹⁰Pb and ¹³⁷Cs profiles. Two cores are representative of the Axios and Aliakmon Rivers depositional environment, whilst the third core represents the Pinios River province; the fourth core represents an environment of outer shelf relict sands. Apparent SARs ranged between 0.667 g cm⁻² yr⁻¹ (Axios and Aliakmon Rivers) and 0.414 g cm⁻² yr⁻¹ (Pinios River). Trawling activities and biomixing are critical processes that may be responsible for the mixing of the surface sediments, as observed from the excess ²¹⁰Pb profiles. The thickness of the surface mixed layer was 4.5 cm in the vicinity of Axios and Aliakmon Rivers and in the area of Pinios River, 3.75 cm on the outer shelf and 1 cm in the area where no trawling was observed. Sediment accumulation appeared to be regulated by variations in the riverine discharge, shelf transport pathways and winnowing processes. Major element variations, such as Si, Al, Ti, V and Ni, were dominated by terrigenous supply as aluminosilicate minerals and quartz, whereas most Ca and Sr were biogenic. Si/Al and Ca/Al ratios have been used to express changes in sediment accumulation and winnowing. Redox processes were depicted by Mn, which showed an increase in the depth of its redoxcline, from 1 cm in inshore stations to 2 cm on the outer shelf. Si/Al ratios follow the Ca/Al ratios and can be used to assess percentage winnowing in the sediment. Increases in these ratios indicate a decrease in sediment input rates and are seen in the upper parts of most of the cores. Anthropogenic or ‘excess’ metal contents have been calculated from Zn/V and Pb/V ratios. Their distributions in the cores showed that by far the highest contamination is associated with the Axios River output, whilst sediments influenced by the Pinios River were relatively uncontaminated.     

Potential toxicity of resuspended particulate matter and sediments: Environmental samples from the Bay of Banyuls-sur-Mer and Thermaikos Gulf

This contribution attempts to determine whether, and to what degree, environmental samples of resuspended particulate matter and sediments exert a toxicological impact. Further, an attempt is made also to screen the toxic level of potentially hazardous sites, based upon established sediment toxicity criteria. Therefore, a rapid, cost-effective and highly sensitive biotest (bioluminescence assay, based upon marine bacteria) has been applied on: biological fluid extracts; bottom sediments; and sediment trap samples. Samples were taken either from the Bay of Banyuls-sur-Mer (northwestern Mediterranean, France) or Thermaikos Gulf (northeastern Mediterranean, Greece). Biological fluid extracts and sediment trap samples corresponded to periods of resuspension events, or preceding and following such events.The results have revealed that the sampling strategy and biotest implemented in this study might be a useful tool for screening the toxicity of resuspended matter and sediments. Resuspension events appear to be able to exert an influence on the chemical forms of the micro-pollutants; thus on their bioavailability and toxicity. Nevertheless, based upon chemical analysis combined with the bioassay, the toxic level of the sediment samples could be determined, with the level at potentially hazardous sites being characterised.     

Hydrodynamics in the Gulf of Aigues-Mortes,NW Mediterranean Sea: In situ and modelling data

The Gulf of Aigues-Mortes (NW Mediterranean Sea) is a midshelf zone whose scale is an intermediate between the nearshore scale (0–10 m depth) and the coastal scale (including the whole continental shelf). Its hydrodynamics is investigated for the first time. ADCP, CTD and thermosalinograph data were collected during three short cruises (HYGAM; March 6–7, 20–21, April 5–6, 2005). They were scheduled approximately every 15 days to sample the gulf circulation under different weather conditions. Moreover, the cruise data were used to validate the Symphonie model, a 3D primitive equations circulation model. The circulation features displayed by in situ data were well reproduced by Symphonie. A downscaling modelling approach was implemented, the largest scale being obtained by the replay of the MFSTEP regional model of the North-Western Mediterranean Sea.     

Bacterial and organic matter distribution in the sediments of the Thracian Sea (NE Aegean Sea)

Recently, black carbon has been introduced as the form of carbon that may be separated from the biologically mediated carbon cycle thereby representing the non-bioavailable fraction of the estimated organic carbon. It has been speculated that the bioavailability of organic matter may be a limiting factor for the presence of active bacteria within the sediments. In order to address this question, marine sediments were collected from the Thracian Sea (Eastern Mediterranean), a complex system impacted by riverine inputs and Black Sea water masses. In addition to counts of total bacteria, we estimated the fraction of active bacteria by using a destaining step to the DAPI staining method. Black carbon was also estimated following the thermal oxidation method in order to determine the fraction of the refractory organic matter. The fraction of black carbon to total organic carbon varied from 16% to 53% indicating that black carbon constitutes a significant pool of sedimentary organic carbon in the Thracian sea. A fraction ranging from 18% to 97% was scored as nucleoid containing cells. We did not record any significant differences in the fraction of nucleoid-containing bacteria among sediment depths (P<0.05) indicating that there was no accumulation of dead bacterial cells with depth. The same was observed for the fraction of black carbon and bioavailable organic carbon with sediment depth (P<0.05) indicating that benthic consumers are not the key regulators of the organic matter pool in these sediments but have a minor effect. A possible reason for these observations and for the uncoupling between the active bacterial fraction and the bioavailability of organic matter could be (i) the presence of refractory components in the estimated bioavailable organic matter and (ii) the hydrological and geological complexity of the study area. The North Aegean marginal slopes are highly unstable experiencing frequent seismic events. These events are capable of inducing sediment transport from the upper slopes thus altering the entire sediment profile. On the other hand, the significant correlations that were recorded between nucleoid-containing cells and phytopigments (chlorophyll a, phaeopigments, chloroplastic pigment equivalents) at all sediment depths indicate that bacterial communities respond immediately to the deposited phytodetritus, using it as a primary source of carbon and energy. Our data suggests that the Thracian Sea sediments are by no means homogeneous and can best be described as a mosaic controlled by numerous local and regional environmental factors.     

Dynamics of particulate major and trace elements in the lower reaches of the Daugava River and adjacent area of the Gulf of Riga (Baltic Sea)

Suspended particulate matter (SPM) was collected in the freshwater-seawater mixing zone in the lower reaches of the Daugava River (Latvia) and adjacent marine area, during five cruises in 1998-2001. The study focused on biogeochemical phase exchange processes. SPM in the freshwater was found to be mainly allochthonous with a high content of organic matter, Mn and sorbed phosphate. Property-salinity plots suggested flocculation of humic-Fe complexes across the salinity gradient. The variability of sorbed phosphate was related to particulate Fe, although no dependence on pH and ionic strength was observed. The Mn contents of SPM mainly follow conservative mixing, but there are also indications of interface exchange of Mn in the mixing zone. The geochemical behaviour of particulate Al appears to differ from that of Fe. In early spring, trace element contents in SPM (Cr, Ni, Cu, Zn, Cd, Pb) correlate tightly with particulate organic matter, whose distribution is linked to phytoplankton distributions.     

Fine particulate organic matter (FPOM) transport and processing in littoral interstices – use of fluorescent markers

A fluorescent labelling method is presented as a new tool for the investigation of organic particle transport and biogenic carbon cycling processes in sandy littoral interstices at Lake Tegel, Berlin, Germany. Passive particle transport through the pore system was studied by in situ exposition of 2.4 μm monodisperse polymeric resin microparticles stained with 7-amino-4-methylcoumarin (AMC). Uptake of fluorescein-5-isothiocyanate (FITC)-labelled Chlorella vulgaris and fine particulate organic matter (FPOM) by the interstitial fauna was investigated in laboratory and field experiments. The major portion (>85%) of the FITC-labelled particles added to sediment cores was recovered from the topmost centimetre of sediment during the study period of 14 days. Uptake of FITC-labelled FPOM was observed in several benthic groups, e.g. chironomids, microcrustaceans, oligochaetes and tardigrads, whereas C. vulgaris was ingested by oligochaetes only. There is evidence to suggest that both are suitable materials for investigating the fragmentation and ingestion of organic material by herbivorous and detritivorous fauna. Field experiments with inert microparticles and FITC-labelled FPOM (<1 mm) prepared from dried alder leaves were carried out in plexiglass tubes as in situ whole core technique. Within the investigation period of two weeks, the transport of FPOM was restricted to the topmost 2–3 cm of sediment in conjunction with a distinct fragmentation to finer size classes with respect to increasing sediment depth. Vertical FPOM transport was hindered by a high interstitial concentration of natural POM and an intensive settlement of the interstices by algae (mainly epispammic algae, 65–96% of algae cell number) and extra-cellular polymeric substances (EPS), which formed a dense three-dimensional structure.     

Toxicity Assessment by Microtox® in Sediments,Pore Waters and Sediment Saline Elutriates in the Gulf of Gdansk (Baltic Sea)

The toxicity of sediments in the Gulf of Gdansk is analyzed in relation to the chemical composition of interstitial and near‐bottom waters, and sediment properties. The toxicity of sediments, pore waters and saline elutriates is determined by using the Microtox® test based on changes in light production of the luminescent bacteria Vibrio fischeri. The results indicate considerable toxicity in the majority of examined sediments. Since the sediment elutriates and pore waters are toxic in some cases, the total toxicity of the sediments is likely to be due to both sediment‐bound and water soluble substances. The sediment toxicity is related to the percentage contribution of the fine fraction of sediments. A significant correlation between the toxicity of the sediments and the black carbon content implies anthropogenic contamination. The toxicity of the sediments is seen to increase with the increase of hydrogen sulfide concentration in pore waters. The ammonia in pore waters was found not to be responsible for the toxicity of the sediments.     

Seasonal distribution of the inorganic carbon system and net ecosystem production in the north eastern shelf of the Gulf of Cádiz (Southwest Iberian Peninsula)

There is scatter information of the inorganic carbon system in the coastal zones and it is important to increment our knowledge and understand the global carbon cycle. We investigated the distribution of inorganic carbon system parameters and its controls in the coastal waters of the north eastern shelf of the Gulf of Cádiz (GoC) during four cruises that took place in June 2006, November 2006, February 2007 and May 2007. The objectives of this study are: (1) to describe the spatio-temporal distribution of inorganic carbon system parameters in waters of the north eastern shelf of the GoC using four cruises, each undertaken in one of the four seasons, (2) to calculate net ecosystem production (NEP) and (3) to examine factors controlling these distributions. The distribution of inorganic carbon system parameters in the north eastern shelf of the Gulf of Cádiz showed temporal and spatial variability. River input, mixing, primary production, respiration, CO₂ air-sea exchange, and remineralization were factors that controlled such distributions. The coastal zone of the GoC is autotrophic on an annual scale at a rate of 1.0 mmol m⁻² d⁻¹. Further measurements are needed it to improve the NEP calculation and to evaluated to intra-annual variability.     

Morphology and composition of spinel in Pu’u ’O’o lava (1996–1998), Kilauea volcano, Hawaii

The morphology and composition of spinel in rapidly quenched Pu’u ’O’o vent and lava tube samples are described. These samples contain glass, olivine phenocrysts (3–5 vol.%) and microphenocrysts of spinel (0.05 vol.%). The spinel surrounded by glass occurs as idiomorphic octahedra 5–50 μm in diameter and as chains of octahedra that are oriented with respect to each other. Spinel enclosed by olivine phenocrysts is sometimes rounded and does not generally form chains. The temperature before quenching was calculated from the MgO content of the glass and ranges from 1150°C to 1180°C. The oxygen fugacity before quenching was calculated by two independent methods and the log f_O2 ranged from −9.2 to −9.9 (delta QFM=−1). The spinel in the Pu’u ’O’o samples has a narrow range in composition with Cr/(Cr+Al)=0.61 to 0.73 and Fe²⁺/(Fe²⁺+Mg)=0.46 to 0.56. The lower the calculated temperature for the samples, the higher the average Fe²⁺/(Fe²⁺+Mg), Fe³⁺ and Ti in the spinel. Most zoned spinel crystals decrease in Cr/(Cr+Al) from core to rim and, in the chains, the Cr/(Cr+Al) is greater in the core of larger crystals than in the core of smaller crystals. The occurrence of chains and hopper crystals and the presence of Cr/(Cr+Al) zoning from core to rim of the spinel suggest diffusion-controlled growth of the crystals. Some of the spinel crystals may have grown rapidly under the turbulent conditions of the summit reservoir and in the flowing lava, and the crystals may have remained in suspension for a considerable period. The rapid growth may have caused very local (μm) gradients of Cr in the melt ahead of the spinel crystal faces. The crystals seem to have retained the Cr/(Cr+Al) ratio that developed during the original growth of the crystal, but the Fe²⁺/(Fe²⁺+Mg) ratio may have equilibrated fairly rapidly with the changing melt composition due to olivine crystallization. Six of the samples were collected on the same day at various locations along a 10-km lava tube and the calculated pre-collection temperatures of the samples show a 5°C drop with distance from the vent. The average Fe²⁺/(Fe²⁺+Mg) of the spinel in these samples shows a weak positive correlation with decreasing MgO in the glass of these samples. The range in Cr₂O₃ (0.041–0.045 wt.%) of the glass for these six samples is too small to distinguish a consistent change along the lava tube. The spinel in the Pu’u ’O’o samples shows a zoning trend in a Cr–Al–Fe³⁺ diagram almost directly away from the Cr apex. This compares with a zoning trend in rapidly quenched MORB samples away from Cr coupled with decreasing Fe³⁺. The trend away from Cr displayed by spinel in rapidly quenched samples is in marked contrast to the trend of increasing Fe³⁺ shown by spinel in slowly cooled lava.     

Spatial distribution and temporal trends of mean precipitation and extremes in the arid region,northwest of China,during 1960–2010

On the basis of daily precipitation records at 76 meteorological stations in the arid region, northwest of China, the spatial and temporal distribution of mean precipitation and extremes were analysed during 1960–2010. The Mann–Kendall trend test and linear least square method were utilized to detect monotonic trends and magnitudes in annual and seasonal mean precipitation and extremes. The results obtained indicate that both the mean precipitation and the extremes have increased except in consecutive dry days, which showed the opposite trend. The changes in amplitude of both mean precipitation and extremes show seasonal variability. On an annual basis, the number of rain days (R0.1) has significantly increased. Meanwhile, the precipitation intensity as reflected by simple daily intensity index (SDII), number of heavy precipitation days (R10), very wet days (R95p), max 1‐day precipitation amount (RX1day) and max 5‐day precipitation amount (RX5day) has also significantly increased. This suggests that the precipitation increase in the arid region is due to the increase in both precipitation frequency and intensity. Trends in extremes are very highly correlated with mean trends of precipitation. The spatial correlation between trends in extremes and trends in the mean is stronger for winter (DJF) than for annual and other seasons. The regional annual and seasonal precipitation and extremes are observed the step jump in mean in the late 1980s. Overall, the results of this study are good indicators of local climate change, which will definitely enhance human mitigation to natural hazards caused by precipitation extremes. Copyright © 2012 John Wiley & Sons, Ltd.     

TEC variations over the Mediterranean before and during the strong earthquake (M = 6.5) of 12th October 2013 in Crete,Greece

In this paper, the total electron content (TEC) data from eight global positioning system (GPS) stations of the EUREF network, provided by IONOLAB (Turkey), were analyzed using discrete Fourier analysis to investigate the TEC variations over the Mediterranean before and during the strong earthquake of 12th October 2013, which occurred west of Crete, Greece. In accordance with the results of similar analyses in the area, the main conclusions of this study are the following: (a) TEC oscillations in a broad range of frequencies occur randomly over an area of several hundred km from the earthquake and (b) high frequency oscillations (f ⩾ 0.0003 Hz, periods T ⩽ 60 m) may point to the location of the earthquake with questionable accuracy. The fractal characteristics of the frequency distribution may point to the locus of the earthquake with higher accuracy. We conclude that the lithosphere–atmosphere–ionosphere coupling (LAIC) mechanism through acoustic or gravity waves could explain this phenomenology.