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1.
We determine the three-dimensional distribution of P- and S-wave velocities for Central São Miguel Island (Azores, Portugal) by tomographic inversion of local earthquake arrival times. We use P- and S-phases from 289 earthquakes recorded by a network of 20 seismometers. The model shows good resolution in the shallowest 5–6 km, as illustrated by different resolution tests. There are several velocity anomalies, interpreted as pyroclastic deposits, intrusive bodies, geothermal fields, and the effects of tectonics. A low Vp zone marks Furnas caldera, probably evidencing volcaniclastic sediments with development of intense geothermal activity. Another low Vp zone extends in correspondence of the highly fractured area between Fogo and the north coast. Conversely, strong positive anomalies are found south of Fogo and northwest of Furnas. They are interpreted in terms of high-density deposits and remnants of a plutonic intrusion. These interpretations are supported by the distribution of Vp/Vs, and are consistent with previous geological, geochemical, and geophysical data.  相似文献   

2.
The deposits of three eruptions in the last 5000 years are described in detail in order to constrain eruptive parameters and allow a quantitative assessment of the hazard from a range of explosive eruption types at Sete Cidades volcano, São Miguel, Azores. These deposits include: the Caldeira Seca eruption (P17) which occurred around 600 yr BP, which was the last explosive event from inside the Sete Cidades caldera, the P11 eruption, dated at 2220 ± 70 yr BP, and the undated P8 eruption (< 3000 yr BP). These deposits were chosen to represent the range of likely explosive activity from the caldera.  相似文献   

3.
Sete Cidades is a central volcano with a summit caldera at the western end of São Miguel Island, Azores. Its stratigraphy comprises two main geological groups: the Inferior Group, the units of which date from more than 200 000 years ago through to 36 000 years before present, consisting of thick lava flows and subaerial volcaniclastic deposits that built the base of the central volcano; and the Superior Group which comprises all the activity from the last 36 000 years, including pumice and scoria fallout and PDC deposits with minor lava flows. The volcanostratigraphy is divided into six main formations — Risco, Ajuda, Bretanha, Lombas, Santa Bárbara and Lagoas, each defined by different activity phases in the volcano's evolution.  相似文献   

4.
Remote islands, such as the São Pedro and São Paulo Archipelago (SPSPA), Brazil, are pristine areas. However, these locations are not exempt from the arrival of anthropogenic agents, such as persistent organic pollutants (POPs). The present study aimed to determine the occurrence and distribution of POPs in the marine biota of the SPSPA. Sample extractions were performed using a microwave-assisted method. The predominant compounds were PCBs and DDTs, which respectively had mean wet weight concentrations of 62.23 and 9.23 ng g−1 in the tropical two-wing flying fish (Exocoetus volitans), 78.66 and 6.81 ng g−1 in the brown booby (Sula leucogaster) and 43.40 and 3.03 ng g−1 in the red rock crab (Grapsus grapsus). Low levels of contaminants suggest a relative degree of isolation. Occurrence and distribution profiles of PCBs support long-range atmospheric transport as the main source of contamination and demonstrate the ubiquity of these pollutants in the marine environment.  相似文献   

5.
We present new 40Ar/39Ar ages and paleomagnetic data for São Miguel island, Azores. Paleomagnetic samples were obtained for 34 flows and one dike; successful mean paleomagnetic directions were obtained for 28 of these 35 sites. 40Ar/39Ar age determinations on 12 flows from the Nordeste complex were attempted successfully: ages obtained are between 0.78 Ma and 0.88 Ma, in contrast to published K–Ar ages of 1 Ma to 4 Ma. Our radiometric ages are consistent with the reverse polarity paleomagnetic field directions, and indicate that the entire exposed part of the Nordeste complex is of a late Matuyama age. The duration of volcanism across São Miguel is significantly less than previously believed, which has important implications for regional melt generation processes, and temporal sampling of the geomagnetic field. Observed stable isotope and trace element trends across the island can be explained, at least in part, by communication between different magma source regions at depth. The 40Ar/39Ar ages indicate that our normal polarity paleomagnetic data sample at least 0.1 Myr (0–0.1 Ma) and up to 0.78 Myr (0–0.78 Ma) of paleosecular variation and our reverse polarity data sample approximately 0.1 Myr (0.78–0.88 Ma) of paleosecular variation. Our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long-term geomagnetic field behavior. Negative inclination anomalies are observed for both the normal and reverse polarity time-averaged field. Within the data uncertainties, normal and reverse polarity field directions are antipodal, but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction.  相似文献   

6.
The new occurrence of the bryozoan Zoobotryon verticillatum Della Chiaje, 1822 is herein recorded in multiple places throughout Azores archipelago. Where introduced, this species has caused important ecological and economical damage and therefore is regarded as invasive. In the Azores, no detrimental effects have yet been noted. The species is so far restricted to marinas (Horta, Faial Island; Vila Franca do Campo, São Miguel Island) and a natural coastal pool located near a secondary harbour (Lajes do Pico, Pico Island).A total of 29 colonies were counted during a specific survey conducted in August 2008 in the marina of Horta. The distribution of the species throughout the eastern and central island groups denotes a wide dispersion area and offers control or eradication measures a low probability of success together with the lack of harbour management practices that could effectively prevent the arrival, settlement and dispersal of non-native species transported by human-assisted means.  相似文献   

7.
Since October 2001, four soil CO2 flux stations were installed in the island of São Miguel (Azores archipelago), at Fogo and Furnas quiescent central volcanoes. These stations perform measurements by the accumulation chamber method and, as the gas flux may be influenced by external variables, the stations are equipped with several meteorological sensors. Multivariate regression analysis applied to the large datasets obtained allowed observing that the meteorological variables may influence the soil CO2 flux oscillations from 18% to 50.5% at the different monitoring sites. Additionally, it was observed that meteorological variables (mainly soil water content, barometric pressure, wind speed and rainfall) play a different role in the control of the gas flux, depending on the selected monitoring site and may cause significant short-term (spike-like) fluctuations. These divergences may be potentially explained by the porosity and hydraulic conductivity of the soils, topographic effects, drainage area and different exposure of the monitoring sites to the weather conditions. Seasonal effects are responsible for long-term oscillations on the gas flux.  相似文献   

8.
The syn-eruptive and post-eruptive history of São Roque tuff cone, its geological setting and volcanological features were studied in detail to understand the role played by the different factors that contributed to the morphological evolution of this relatively simple and small volcanic edifice.In addition, attention was also focused on the series of natural changes that affected the tuff cone during the course of the years and that finally led to its structural disassembly. A novel model is proposed to explain this process.The São Roque volcanic centre, located on the island of São Miguel (Azores), consists of two well-consolidated bodies and numerous small islets that formed more than 4700 years ago during the hydromagmatic activity that took place along an intruding dyke, whose NNW–SSE trend is in agreement with the regional tectonic pattern. The eruptive vents probably migrated progressively from SSE to NNW, forming small edifices through the rapid accumulation of sediments during alternating phases of “dry” and “wet” magmatic emissions. Syn-eruptive partial collapses greatly modified the original morphological structure of these edifices, probably allowing sea water to continuously flow into the vents. The complex interaction of these factors controlled the depth of magma fragmentation, producing different types of deposits, in which the ash-lapilli ratio varies considerably. The high-water saturation degree of these deposits caused syn-eruptive and post-eruptive remobilization which resulted in collapses and some small-scale landslides.Post-eruptive, WNW–ESE trending transtensional and extensional tectonic activities operated during the initial dissection of the cone, generating instability. Furthermore, the rapid accumulation of “wet” tephra, and its following consolidation, caused selective collapses that favoured the fragmentation of the deposit and caused the formation of numerous islets separated by radially-arranged channels. Collapses also involved the lava units emplaced in more recent times around the tuff cone, which show that brittle deformation has been significant in the area for a prolonged period.  相似文献   

9.
Rare earths (RE) in basalts erupted within the rift of the Mid-Atlantic Ridge show a progressive change from light-RE enriched to depleted patterns from the Azores Platform (40°N) down to 33°30′N. South, the pattern remains light-RE depleted as along other “normal ridge” segments. A progressive increase in chemical variability of the basalts towards the Azores is also noted.The latitudinal RE profile and corresponding ΣFeO/ΣFeO + MgO variations, together, indicate that the origin of these basalts cannot be accounted for simply by considering variable extents of partial melting of a single mantle source and subsequent fractional crystallization during the ascent of the magmas. These two processes produce only second-order effects on the RE patterns. The data requires the presence of a distinct, light-RE richer, mantle source beneath the Azores Platform relative to that of south of 33°30′N and an intermediate zone where both mantle types mix. The relative contribution of the Azores mantle source to the mix appears to decrease fairly regularly southward along the ridge and becomes negligible at 33°30′N. Increasing chemical variability of the basalts towards the Azores is probably caused by correspondingly larger extent of fractional crystallization at shallow depth, and/or greater variability in the extent of partial melting, apparently subsequent to, and superimposed on the mixing of the two mantle sources.The combined morphological, geophysical and RE evidence along the profile are consistent with a model suggesting upwelling of a major blob (plume) under the Azores Plateau; and reveal the present extent of the blob's overflow and mixing with the asthenosphere depleted in large ionic lithophile trace elements. The influence of the Azores blob is geochemically detectable up to 1000 km southwestward beneath the ridge axis.  相似文献   

10.
Here I present textural data (i.e., vesicularity, vesicle size distributions (VSD), plagioclase crystallinity, crystal size distributions (CSD), combined with fractal analyses of particle outlines) from a natural succession of alternating fall and surge deposits in the emergent Capelas tuff cone (Azores).The textural variation in the Capelas succession is surprisingly small considering the wide variety of fragmentation processes, vent activity and emplacement mechanisms that are characteristic of emergent eruptions. The plagioclase crystal content varies between 24 and 33 vol.%. CSD analyses of plagioclase show near-linear trends with a slight increase in time for the smallest crystal sizes (with surge deposits having more groundmass plagioclase when compared with fall deposits). This is consistent with crystallization induced by degassing and decompression at lower eruption rates. The vesicularities of the Capelas pyroclasts are more variable (18 to 59 vol.%), with VSDs displaying kinked trends characteristic of coalescence. This is especially evident in the fall deposits, and consistent with being formed in continuous uprush (jetting) with an overall shallow fragmentation level within the conduit. Bubble coalescence can also be identified in the surge deposits, although to a much lesser extent. The amount of bubble coalescence is negatively correlated with the amount of groundmass crystallization (i.e., plagioclase) in the Capelas deposits.A relatively broad range of fractal dimensions (with average Dbox = 1.744 and σ = 0.032) for the outlines of pyroclastic fragments emplaced by fall or as surges indicate that there is little difference in the fragmentation process itself at Capelas. In addition to this, the fact that the fractal dimensions for both the fall and surge end-members completely overlap suggests that shape modification due to abrasion and chipping of grain edges was minor during emplacement of base surges. These results are consistent with emergent eruptions, building tuff cones, to be a relatively low-energy phreatomagmatic landform (e.g., at least when compared with more energetic phreatomagmatic eruptions producing tuff rings and maar volcanoes).  相似文献   

11.
The major tectonic elements of the Azores triple junction have been mapped using long-range side-scan sonar. The data enable the Mid-Atlantic Ridge axis to be located with a precision of a few kilometres. Major faults and other tectonic and volcanic elements of the ridge maintain their regional trend of 010° to 020° past the triple junction area. There is no oblique spreading, and only minor transform offsets of the Mid-Atlantic Ridge occur here. The main effect of the triple junction or Azores hot spot is to diminish the amplitude of the median valley to 200 m or less. There is no axial high: a topographic high seen on several profiles is located to the east of the Mid-Atlantic Ridge spreading axis and does not appear to have any fundamental significance.The third arm of the triple junction includes the Azores srreading centre which appears to have developed as a series of en echelon rifted basins (the Terceira Rift) extending from Formigas Trough at 36.8°N, 24.5°W to a point near 39.3°N, 28.8°W. There are indications that recent activity in the spreading centre may be concentrated in a series of ridges which flank the older rifted basins. Until recently the northwest end of the Terceira Rift was connected to the Mid-Atlantic Ridge axis either directly at an RRR junction, or via a transform fault. The triple junction has probably moved south during the last 6 Ma to a positin on the Mid-Atlantic Ridge near 38.7°N.Initiation of the Azores spreading centre may have occurred during the 36 Ma B.P. rearrangement of poles, with an RFF triple junction north from the East Azores fracture zone to the North Azores fracture zone and transferring a wedge of European plate to the African plate.The tectonic elements revealed by this study are in good agreement with inferred earthquake mechanisms and with the RM2 plate tectonic model of Minster and Jordan, but east-west motion between North America and Africa does not seem to be compatible with the other motions at the triple junction unless it is of very recent (2>3 Ma) origin.  相似文献   

12.
Two years of altimetric data from Topex/Poseidon (October 1992-September 1994) and ERS-1 (October 1992-December 1993) were assimilated into a numerical model of the North Atlantic. The results of these simulations are analysed in the Azores region to assess the performance of our model in this particular region. Maps of instantaneous dynamic topography and transports show that the model performs well in reproducing the velocities and transports of the Azores Front. Drifter data from the Semaphore experiment are also used to study the correlation between the drifter velocities and the corresponding model velocities. Some interesting oceanographic results are also obtained by examining the seasonal and interannual variability of the circulation and the inuence of bathymetry on the variability of the Azores Front. Thus, on the basis of our two year experiment, it is possible to confirm the circulation patterns proposed by previous studies regarding the seasonal variations in the origin of the Azores Current. Moreover, it is shown that the Azores Current is quite narrow in the first year of assimilation (1992±1993), but becomes much wider in the second year (1993±1994). The role of the bathymetry appears important in this area since the mesoscale activity is shown to be strongly related to the presence of topographic slopes. Finally, spectral analyses of sea-level changes over time and space are used to identify two types of wave already noticed in other studies: a wave with (300 km)–1 wave number and (120 days)–1 frequency, which is characteristic of mesoscale undulation, and a wave with (600 km)–1 wave number and (250 days)–1 frequency which probably corresponds to a Rossby wave generated in the east of the basin.  相似文献   

13.
—The development of the digital seismic network in the Azores Archipelago during recent years made it possible to obtain the amplitudes (waveform) of recorded motion in a large set of stations. With this new data, maximum amplitudes of the Wood Anderson seismograph are computed, for each station/component, which, together with epicentral distances, allows for the estimation of local magnitude M L . We used data recorded in 8 digital permanent three-component stations, with inter-stations distances up to 300 km, in the period June 1998 – June 2000, corresponding to a set of 1315 events with magnitude (M L or M D ) 2<M<5.8 and epicenters located in the Azores region, to estimate the coefficients of the equation to compute M L , as well as to determine the corrections to be applied to each station. The new set of parameters, formed by attenuation coefficients and station corrections, were introduced in the calculations of the M L , leading to smaller dispersions in the analyzed dataset. We also conclude that the attenuation in the first 150 km is similar to the California values, although higher for longer distances.  相似文献   

14.
Shallow-water hydrothermal activity is widely present at Azores archipelago. Organisms in such environments present great potential as sentinels of the effects derived from chronically exposure to increased temperature, metal concentrations and reduced pH. This study aimed to evaluate metal concentration in Corallina elongata collected at locations exposed and not exposed to shallow-water hydrothermal activity and evaluate changes in its calcareous structure. Elemental concentration was determined and morphometric analysis was performed by scanning electron microscopy. Thicker cell walls and a bleached appearance were observed on C. elongata specimens from the hydrothermally active location, as well as increased concentrations of elements associated to volcanic activity.This study reports on metal accumulation and morphometric changes in the calcareous structure of C. elongata from a hydrothermally active location, adding new data for further research on such habitats and communities, providing an insight on how coralline algae might be affected by ocean acidification.  相似文献   

15.
Little is known about the spatial and temporal variability of the stable isotopic composition of precipitation in the North Atlantic and its relationship to the North Atlantic Oscillation (NAO) and anthropogenic climate change. The islands of the Azores archipelago are uniquely positioned in the middle of the North Atlantic Ocean to address this knowledge gap. A survey of spatial and temporal variability of the stable isotope composition of precipitation in Azores is discussed using newly presented analyses along with Global Network of Isotopes in Precipitation data. The collected precipitation samples yield a new local meteoric water line (δ2H = 7.1 * δ18O + 8.46) for the Azores region and the North Atlantic Ocean. The annual isotopic mean of precipitation shows a small range for the unweighted and precipitation mass‐weighted δ18O‐H2O values. Results show an inverse relation between the monthly δ18O‐H2O and the amount of precipitation, which increases in elevation and into the interior of the island. Higher amounts of precipitation (from convective storm systems) do not correspond to the most depleted values of stable isotopes in precipitation. Precipitation shows an orographic effect with depleted δ18O‐H2O values related to the Rayleigh effect. Monthly δ18O‐H2O values for individual precipitation sampling stations show little relationship to air temperature. Results show a local source of moisture during the summer with the characteristics of the first vapour condensate. The stable isotope composition of precipitation is strongly correlated to the NAO index, and δ18O‐H2O values show a statistically significantly trend towards enrichment since 1962 coincident with the increased air temperature and relative humidity due to climate change. Results are in line with observations of increasing sea surface temperature and relative humidity.  相似文献   

16.
The Azores archipelago is one of the most active volcanic areas in the North Atlantic region, with approximately 30 eruptions during the last 600 years. The geochemical composition of associated tephra-derived glass is, however, not well characterized. This study presents major element compositions of glass shards from five major eruptives on the Azores: a trachybasaltic eruptive on the island of Faial (Capelinhos AD, 1957) and four explosive trachytic eruptives on the island of São Miguel (Fogo A c. 5600 cal yrs. BP, Sete Cidades c. AD 1440, Fogo AD 1563 and Furnas AD 1630). The major element compositions suggest that tephras from three active stratovolcanoes on São Miguel, Sete Cidades, Fogo and Furnas, can be distinguished from one another using bi-plots of FeOtot vs. TiO2 and FeOtot vs. CaO. Late Holocene tephras found on Ireland have previously been attributed to eruptions occurring on Jan Mayen but possess a strong geochemical similarity to proximal tephras from the Azores, especially those from the Furnas volcano. The similarity of the proximal tephras on São Miguel, especially Furnas AD 1563 and Furnas AD 1630 and distal tephras in Ireland is demonstrated by strong similarity coefficients (>0.95) and the closeness of major element composition. The dominant wind direction over the Azores is favourable for tephra dispersal to western Europe and we suggest that at least three tephras found in Ireland were erupted from the Furnas volcano, and that trachytic tephras erupted from explosive eruptions on São Miguel have a potential to contribute to the construction of a European-wide tephrostratigraphic framework.  相似文献   

17.
—?A 2-D and a 3-D finite element representation using Drucker-Prager cap model is employed in the study to determine the seismic response of a volcanic hill located in one of the islands in the Azores Archipelago. In order to test the applicability of these models we used the motion recorded at the base of the hill during an aftershock of the July 9, 1998 earthquake and compared the numerical response with the record obtained at the top of the hill. Several comparisons and sensitivity analyses were made to identify the most important dynamic parameters influencing the response. Even though the match is not yet adequate for any one of the representations, especially in time domain, the 3-D model showed a good fitting in terms of Fourier Spectrum. Up to a PGA of 0.24?g the behavior of the hill is approximately linear, with higher amplifications going upwards along a vertical interior column; beyond this limit, there is a clear nonlinear behavior.  相似文献   

18.
v--vAn essential step in the establishment of an International Monitoring System (IMS) infrasound station is the site survey. The survey seeks a location with relatively low infrasonic noise and the necessary logistical support. This paper reports results from our surveys of two of the oceanic sites in the IMS - the Azores and Cape Verde. Each survey sampled infrasonic noise, wind velocity, air temperature and humidity for ~3 weeks at 4 sites near the nominal IMS locations. The surveys were conducted on Sao Miguel (the main island in the Azores) and Maio (Cape Verde). Infrasonic noise was measured using the French MB2000 microbarometer.¶During our 3-week experiment in January the trade winds at Cape Verde varied little from an azimuth of 63°. Because of the unvarying wind azimuth, the experiment gave us an opportunity to examine the effectiveness of a forest at reducing both wind speed and infrasonic noise. We find that the thick Acacia forest on Maio reduces wind speeds at a 2 m elevation by more than 50% but does not reduce infrasonic noise at frequencies below 0.25 Hz. This forest serves as a high-frequency filter and clearly does not reduce long-period noise levels which are due to large-scale turbulence in the atmospheric boundary layer above the forest. This is consistent with our observations in the Azores where the relationship between infrasonic noise and wind speed is more complex due to frequent changes in wind azimuth.¶In Cape Verde, wind speed and infrasonic noise are relatively constant. The diurnal variations are clearly seen however the microbarom is only rarely sensed. In the Azores, during our 3-week experiment in November and December of 1998, wind speed and infrasonic noise change rapidly. At this location, daily noise level swings of 40 to 50 dB at 0.1 Hz are not uncommon in the early winter and are due to changes in wind speed and atmospheric turbulence. The effectiveness of an infrasound station in the Azores will be strongly dependent on time during the winter season.¶The two surveys illustrate some of the difficulties inherent in the selection of sites for 1 to 3 km aperture arrays on oceanic islands. Due to elevated noise levels at these sites, 8 element, 2 km aperture arrays are strongly preferred.  相似文献   

19.
Six volcanic zones comprise São Miguel, the largest island in the Azores. All are Quaternary in age except the last, which is partly Pliocene. From west to east the zones are (1) the trachyte stratovolcano of Sete Cidades, (2) a field of alkali-basalt cinder cones and lava flows with minor trachyte, (3) the trachyte stratovolcano of Agua de Pau, (4) a field of alkali-basalt cinder cones and lava flows with minor trachyte and tristanite, (5) the trachyte stratovolcano of Furnas, and (6) the Nordeste shield, which includes the Povoação caldera and consists of alkali basalt, tristanite, and trachyte. New radiocarbon and K-Ar ages augment stratigraphic data obtained during recent geologic mapping of the entire island and provide improved data to interpret eruption frequency. Average dormant intervals for the past approximately 3000 years in the areas active during that time are about 400 years for Sete Cidades, 145 for zone 2, 1150 for Agua de Pau, and 370 for Furnas. However, the average dormant interval at Sete Cidades increased from 400 to about 680 years before each of the past two eruptions, and the interval at Furnas decreased from 370 to about 195 years before each of the past four eruptions. Eruptions in zone 4 occurred about once every 1000 years during latest Pleistocene and early Holocene time; none has occurred for about 3000 years. The Povoação caldera truncates part of the Nordeste shield and probably formed during the middle to late Pleistocene. Calderas formed during latest Pleistocene time at the three younger stratovolcanoes in the sequence: outer Agua de Pau (between 46 and 26.5 ka), Sete Cidades (about 22 ka), inner Agua de Pau (15.2 ka), and Furnas (about 12 ka). Normal faults are common, but many are buried by Holocene trachyte pumice. Most faults trend northwest or west-northwest and are related to the Terceira rift, whose most active segment on São Miguel passes through Sete Cidades and zone 2. A major normal fault displaces Nordeste lavas 150–250 m and may mark the location of an ancestral Terceira rift. Recent seismicity (e.g., in the 1980s) generally has been scattered, but some small earthquake swarms have occurred beneath the north-eastern flank of Agua de Pau.  相似文献   

20.
The State of São Paulo is the most developed area in Brazil and was impacted by persistent organic pollutants for several decades. This study investigated organochlorines in five species of small cetaceans (Pontoporia blainvillei, Stenella frontalis, Sotalia guianensis, Tursiops truncatus and Steno bredanensis) found dead along the coast of São Paulo between 1997 and 2003. DDTs (15.9 μg g−1 lipid; mean for all pooled individuals) and PCBs (8.08 μg g−1) exhibited the highest concentrations in the animals, reflecting large amounts formerly used in Brazil. Lower levels of mirex (0.149 μg g−1), HCB (0.051 μg g−1), CHLs (0.008 μg g−1) and HCHs (0.007 μg g−1) were detected in all species. Residual pattern of DDTs in dolphins suggests that o,p′-DDT is more recalcitrant than p,p′-DDT in the body of the animals and/or the environment. In contrast to p,p′-DDT, residues of o,p′-DDT seem to be preferentially converted into o,p′-DDD rather than o,p′-DDE.  相似文献   

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