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1.
Liverpool Bay is a region of freshwater influence which receives significant freshwater loading from a number of major English
and Welsh rivers. Strong tidal current flow interacts with a persistent freshwater-induced horizontal density gradient to
produce strain-induced periodic stratification (SIPS). Recent work (Palmer in Ocean Dyn 60:219–226, 2010; Verspecht et al. in Geophys Res Lett 37:L18602, 2010) has identified significant modification to tidal ellipses in Liverpool Bay during stratification due to an associated reduction
in pycnocline eddy viscosity. Palmer (Ocean Dyn 60:219–226, 2010) identified that this modification results in asymmetry in flow in the upper and lower layers capable of permanently transporting
freshwater away from the Welsh coastline via a SIPS pumping mechanism. Observational data from a new set of observations from
the Irish Sea Observatory site B confirm these results; the measured residual flow is 4.0 cm s−1 to the north in the surface mixed layer and 2.4 cm s−1 to the south in the bottom mixed layer. A realistically forced 3D hydrodynamic ocean model POLCOMS succeeds in reproducing
many of the characteristics of flow and vertical density structure at site B and is used to estimate the transport of water
through a transect WT that runs parallel with the Welsh coast. Model results show that SIPS is the dominant steady state,
occurring for 78.2% of the time whilst enduring stratification exists only 21.0% of the year and enduring mixed periods, <1%.
SIPS produces a persistent offshore flow of freshened surface water throughout the year. The estimated net flux of water in
the surface mixed layer is 327 km3 year
−1, of which 281 km3 year−1 is attributable to SIPS periods. Whilst the freshwater component of this flux is small, the net flux of freshwater through
WT during SIPS is significant, the model estimates 1.69 km3 year−1 of freshwater to be transported away from the coast attributable to SIPS periods equivalent to 23% of annual average river
flow from the four catchment areas feeding Liverpool Bay. The results show SIPS pumping to be an important process in determining
the fate of freshwater and associated loads entering Liverpool Bay. 相似文献
2.
An automatic scheme for baseline correction of strong-motion records in coseismic deformation determination 总被引:2,自引:0,他引:2
Coseismic deformation can be determined from strong-motion records of large earthquakes. Iwan et al. (Bull Seismol Soc Am
75:1225–1246, 1985) showed that baseline corrections are often required to obtain reliable coseismic deformation because baseline offsets lead
to unrealistic permanent displacements. Boore (Bull Seismol Soc Am 91:1199–1211, 2001) demonstrated that different choices of time points for baseline correction can yield realistically looking displacements,
but with variable amplitudes. The baseline correction procedure of Wu and Wu (J Seismol 11:159–170, 2007) improved upon Iwan et al. (Bull Seismol Soc Am 75:1225–1246, 1985) and achieved stable results. However, their time points for baseline correction were chosen by a recursive process with
an artificial criterion. In this study, we follow the procedure of Wu and Wu (J Seismol 11:159–170, 2007) but use the ratio of energy distribution in accelerograms as the criterion to determine the time points of baseline correction
automatically, thus avoiding the manual choice of time points and speeding up the estimation of coseismic deformation. We
use the 1999 Chi-Chi earthquake in central Taiwan and the 2003 Chengkung and 2006 Taitung earthquakes in eastern Taiwan to
illustrate this new approach. Comparison between the results from this and previous studies shows that our new procedure is
suitable for quick and reliable determination of coseismic deformation from strong-motion records. 相似文献
3.
We address possibilities of minimising environmental risks using statistical features of current-driven propagation of adverse
impacts to the coast. The recently introduced method for finding the optimum locations of potentially dangerous activities
(Soomere et al. in Proc Estonian Acad Sci 59:156–165, 2010) is expanded towards accounting for the spatial distributions of probabilities and times for reaching the coast for passively
advecting particles released in different sea areas. These distributions are calculated using large sets of Lagrangian trajectories
found from Eulerian velocity fields provided by the Rossby Centre Ocean Model with a horizontal resolution of 2 nautical miles
for 1987–1991. The test area is the Gulf of Finland in the northeastern Baltic Sea. The potential gain using the optimum fairways
from the Baltic Proper to the eastern part of the gulf is an up to 44% decrease in the probability of coastal pollution and
a similar increase in the average time for reaching the coast. The optimum fairways are mostly located to the north of the
gulf axis (by 2–8 km on average) and meander substantially in some sections. The robustness of this approach is quantified
as the typical root mean square deviation (6–16 km) between the optimum fairways specified from different criteria. Drastic
variations in the width of the ‘corridors’ for almost optimal fairways (2–30 km for the average width of 15 km) signifies
that the sensitivity of the results with respect to small changes in the environmental criteria largely varies in different
parts of the gulf. 相似文献
4.
Transport properties in small-scale coastal flows: relative dispersion from VHF radar measurements in the Gulf of La Spezia 总被引:2,自引:2,他引:0
Angelique C. Haza Tamay M. Özgökmen Annalisa Griffa Anne Molcard Pierre-Marie Poulain Germana Peggion 《Ocean Dynamics》2010,60(4):861-882
Lagrangian transport characteristics in the Gulf of La Spezia, a 5 × 10-km area along the western coast of Italy, are investigated
using data collected from a very high frequency (VHF) radar system with 250 m and 30-min resolution and two clusters of Coastal
Dynamics Experiment surface drifters during 2 weeks in the summer of 2007. The surface drifters are found to follow the temporal
and spatial evolution of the finite-scale Lyapunov exponents (FSLEs) computed by the VHF radar, indicating the precision of
both the radar measurements and the diagnostic FSLE in mapping accurately the transport pathways. In light of this agreement,
an analysis of the relative dispersion is conducted. It is found that the average FSLE value varies within a narrow range
of 4 day-1 £ l £ 7 day-14 \;\mbox{day}^{-1} \leq \lambda \leq 7 \;\mbox{day}^{-1} and displays an exponential regime over the entire extent of the measurements. The dynamical implication is that relative
dispersion is controlled nonlocally, namely by slow, persistent, energetic mesoscale structures as opposed to the rapidly
evolving high-gradient small-scale turbulent features. The value of the exponent is about an order of magnitude larger than
those found in previous modeling studies and analysis of SCULP data in the Gulf of Mexico but somewhat smaller than that estimated
from CLIMODE drifters in the Gulf Stream region. Scaling of the FSLE using a metric of resolved gradients of the Eulerian
fields in the form of a positive Okubo–Weiss criterion is useful, but not as precise as in modeling studies. The horizontal
flow convergence is found to have a small yet tangible effect on relative dispersion. 相似文献
5.
We present thermal measurements made by high spatial resolution ground-based (a hand-held thermal camera) and low spatial
resolution space-based (MODIS) instruments for a lava flow field active during the last phase of the May–July 2003 eruption
at Piton de la Fournaise (La Réunion). Multiple oblique ground-based thermal images were merged to provide full coverage of
the flow-field. These were then corrected for path length attenuation and orthorectified, allowing the at-surface radiance
emitted by the flow-field to be estimated. Comparison with the radiance recorded by the MODIS sensors during the eruption
reveals that, for clear-sky conditions and moderate-to-low viewing angles (satellite zenith <40°), the satellite measurements
represent ∼90% of the at-surface radiance, and thus represent valuable data for quantifying volcanic thermal anomalies. Nevertheless,
extreme viewing geometries and the presence of clouds strongly affect the radiance reaching the sensor and affected data from
94% of the overpasses. Ground-based thermal data were used to investigate an empirical relationship between the radiant heat
flux and lava discharge rate during the emplacement of pahoehoe flows. While the average radiation temperature for flow surface
that were 6–24 h old ranged between 500 K and 625 K, the ratio between radiative heat flux and Time-Averaged lava Discharge
Rate (TADR) ranged between 1.5 × 108 J m−3 and 3.5 × 108 J m−3. This relationship was used to estimate TADR values from optimal MODIS data and produced results in line with those obtained
from GPS surveys (Coppola et al. 2005). Our results underscore the importance of ground-based thermal analysis for the interpretation of satellite measurements,
particularly in terms of calculating discharge rate trends. 相似文献
6.
E. Porcu P. Gregori J. Mateu 《Stochastic Environmental Research and Risk Assessment (SERRA)》2007,21(6):683-693
There is a great demand for statistical modeling of phenomena that evolve in both space and time, and thus, there is a growing
literature on correlation function models for spatio-temporal processes. In particular, various properties of these correlation
functions have been studied only for the merely spatial or temporal case, fact that constitutes a strong motivation for our
work. The goal of this paper is to inspect some properties, obtained with respect to partial differentiation and integration,
of stationary spatio-temporal correlation functions for which anisotropy is obtained through isotropy between components as
in Fernández-Casal et al. (Stat Comput 13(2):127–136, 2003). We show that through partial differentiation and integration it is possible to obtain permissible spatio-temporal correlation
functions in the space–time domain. Other new results regard specific classes of space–time correlations introduced in recent
literature. A curious result arises by differentiating scale mixtures of Euclid’s hat.
Work partially funded by grant MTM2004-06231 from the Spanish Ministery of Science and Education. 相似文献
7.
D. R. Helwade A. Subramanyam 《Stochastic Environmental Research and Risk Assessment (SERRA)》2010,24(2):271-281
In this paper, a certain bivariate exponential distribution is used for the spatial prediction. The unobserved random variable
is predicted by the projection onto the space of all linear combinations of the powers, up to degree m, of the observed random variables plus the constant 1. We obtain a solution by assuming that all the bivariate distributions
follow Gumbel’s type III or logistic form of bivariate exponential. The method is implemented on two data sets and the results
are presented. The predictions are compared with the original values through Mean Structural Similarity (MSSIM) index of Wang
et al. (IEEE Trans Image Process 13(4):600–612, 2004). Using the MSSIM index the proposed method is also compared with Ordinary Kriging and with Simple Kriging after normal score
transform. 相似文献
8.
Use of spectral acceleration data for determination of three-dimensional attenuation structure in the Pithoragarh region of Kumaon Himalaya 总被引:1,自引:0,他引:1
Three-dimensional attenuation structures are related to the subsurface heterogeneities present in the earth crust. An algorithm
for estimation of three-dimensional attenuation structure in the part of Garhwal Himalaya, India has been presented by Joshi
(Curr Sci 90:581–585, 2006b; Nat Hazards 43:129–146, 2007). In continuation of our earlier approach, we have presented a method in which strong motion data have been used to estimate
frequency-dependent three-dimensional attenuation structure of the region. The border district of Pithoragarh in the Higher
Himalaya, India, lies in the central seismic gap region of Himalaya. This region falls in the seismic zones IV and V of the
seismic zoning map of India. A dense network consisting of eight accelerographs has been installed in this region. This network
has recorded several local events. An algorithm based on inversion of strong motion digital data is developed in this paper
to estimate attenuation structure at different frequencies using the data recorded by this network. Twenty strong motion records
observed at five stations have been used to estimate the site amplification factors using inversion algorithm defined in this
paper. Site effects obtained from inversion has been compared with that obtained using Nakamura (1988) and Lermo et al. (Bull Seis Soc Am 83:1574–1594, 1993) approach. The obtained site amplification term has been used for correcting spectral acceleration data at different stations.
The corrected spectral acceleration data have been used as an input to the developed algorithm to avoid effect of near-site
soil amplification term. The attenuation structure is estimated by dividing the entire area in several three-dimensional block
of different frequency-dependent shear wave quality factor Q
β
(f). The input to this algorithm is the spectral acceleration of S phase of the corrected accelerogram. The outcome of the algorithm
is given in terms of attenuation coefficient and source acceleration spectra. In the present study, this region has been divided
into 25 rectangular blocks with thickness of 10 km and surface dimension of 12.5 × 12.1 km, respectively. Present study gives
three-dimensional attenuation model of the region which can be used for both hazard estimation and simulation of strong ground
motion. 相似文献
9.
Modeling and observations of high-frequency flow variability and internal waves at a Caribbean reef spawning aggregation site 总被引:2,自引:2,他引:0
The characteristics and forcing mechanisms of high-frequency flow variations (periods of minutes to days) were investigated
near Gladden Spit, a reef promontory off the coast of Belize. Direct field observations and a high-resolution (50-m grid size)
numerical ocean model are used to describe the flow variations that impact the initial dispersion of eggs and larvae from
this site, which serves as a spawning aggregation site for many species of reef fishes. Idealized sensitivity model experiments
isolate the role of various processes, such as internal waves, wind, tides, and large-scale flow variations. The acute horizontal
curvature and steep topography of the reef intensify the flow, create small-scale convergence and divergence zones, and excite
high-frequency oscillations and internal waves. Although the tides in this area are relatively small (∼10-cm amplitude), the
model simulations show that tides can excite significant high-frequency flow variations near the reef, which suggests that
the preference of fish to aggregate and spawn in the days following the time of full moon may not be coincidental. Even small
variations in remote flows (2–5 cm s−1) due to say, meso-scale eddies, are enough to excite near-reef oscillations. Model simulations and the observations further
suggest that the spawning site at the tip of the reef provides initial strong dispersion for eggs, but then the combined influence
of the along-isobath flow and the westward wind will transport the eggs and larvae downstream of Gladden Spit toward less
turbulent region, which may contribute to enhanced larval survival. 相似文献
10.
Sébastien Blaise Richard Comblen Vincent Legat Jean-François Remacle Eric Deleersnijder Jonathan Lambrechts 《Ocean Dynamics》2010,60(6):1371-1393
We describe the space discretization of a three-dimensional baroclinic finite element model, based upon a discontinuous Galerkin
method, while the companion paper (Comblen et al. 2010a) describes the discretization in time. We solve the hydrostatic Boussinesq equations governing marine flows on a mesh made
up of triangles extruded from the surface toward the seabed to obtain prismatic three-dimensional elements. Diffusion is implemented
using the symmetric interior penalty method. The tracer equation is consistent with the continuity equation. A Lax–Friedrichs
flux is used to take into account internal wave propagation. By way of illustration, a flow exhibiting internal waves in the
lee of an isolated seamount on the sphere is simulated. This enables us to show the advantages of using an unstructured mesh,
where the resolution is higher in areas where the flow varies rapidly in space, the mesh being coarser far from the region
of interest. The solution exhibits the expected wave structure. Linear and quadratic shape functions are used, and the extension
to higher-order discretization is straightforward. 相似文献
11.
The wave-induced velocity and pressure fields beneath a large amplitude internal solitary wave of depression propagating over
a smooth, flat, horizontal, and rigid boundary in a shallow two-layer fluid are computed numerically. A numerical ocean model
is utilised, the set-up of which is designed and tuned to replicate the previously published experimental results of Carr
and Davies (Phys Fluids 18(1):016,601–1–016,601–10, 2006). Excellent agreement is found between the two data sets and, in particular, the numerical simulation replicates the finding
of a reverse flow along the bed aft of the wave. The numerically computed velocity and pressure gradients confirm that the
occurrence of the reverse flow is a consequence of boundary layer separation in the adverse pressure gradient region. In addition,
vortices associated with the reverse flow are seen to form near the bed. 相似文献
12.
A reappraisal of shear wave splitting parameters from Italian active volcanic areas through a semiautomatic algorithm 总被引:1,自引:0,他引:1
Shear wave splitting parameters represent a useful tool to detail the stress changes occurring in volcanic environments before
impending eruptions. In the present paper, we display the parameter estimates obtained through implementation of a semiautomatic
algorithm applied to all useful datasets of the following Italian active volcanic areas: Mt. Vesuvius, Campi Flegrei, and
Mt. Etna. Most of these datasets have been the object of several studies (Bianco et al., Annali di Geofisica, XXXXIX 2:429–443,
1996, J Volcanol Geotherm Res 82:199–218, 1998a, Geophys Res Lett 25(10):1545–1548, 1998b, Phys Chem Earth 24:977–983, 1999, J Volcanol Geotherm Res 133:229–246, 2004, Geophys J Int 167(2):959–967, 2006; Del Pezzo et al., Bull Seismol Soc Am 94(2):439–452, 2004). Applying the semiautomatic algorithm, we confirmed the results obtained in previous studies, so we do not discuss in much
detail each of our findings but give a general overview of the anisotropic features of the investigated Italian volcanoes.
In order to make a comparison among the different volcanic areas, we present our results in terms of the main direction of
the fast polarization (φ) and percentage of shear wave anisotropy (ξ). 相似文献
13.
Earthquake damage data is of vital importance both as an indicator of the likely performance of buildings in future earthquakes,
as well as for the calibration of existing theoretical-analytical models regarding building vulnerability. The analysis of
damage data collected shortly after the August 14, 2003 Lefkada Island, Greece earthquake revealed the higher vulnerability
of masonry buildings vis-à-vis all other building typologies on the island. However, promising means in strengthening the
existing masonry stock emerge, when considering the improved performance of buildings of a dual-system of stone-masonry and
timber frame – a construction practice uniquely adopted in the island. Based on the parameterless seismic intensity scale
(psi) proposed by Spence et al. (1991) a set of preliminary vulnerability curves for the typologies of the island’s buildings are proposed. 相似文献
14.
Matthew Robert Palmer 《Ocean Dynamics》2010,60(2):219-226
Understanding the fate of freshwater runoff and corresponding nutrient and pollution loads is of critical importance for the
development of accurate predictive models and coastal management tools. A key element of such studies is the identification
and understanding of the interaction between stratification and current structure. This paper presents a new series of measurements
made in the Liverpool Bay region of freshwater influence (ROFI) during spring 2004 where freshwater-maintained horizontal
density gradients and strong tidal currents interact to produce strain-induced periodic stratification (SIPS). During stratification,
tidal current profiles are significantly modified such that the tidal flow deviates from the otherwise rectilinear E–W axis
generating counter rotating upper and lower mixed layers. This feature has often been reported for the Rhine ROFI but not
previously identified in Liverpool Bay despite previous investigation at this site. Investigation of an ongoing long-term
dataset collected nearby reveals this process to be a common feature throughout the year. Liverpool Bay is shown to maintain
three different regimes, long term mixed, long term stratified, and a transitional state when SIPS occurs. The phase of SIPS
relative to the tide results in a residual flow away from the Welsh coastline in the upper water column of 2.3–3.6 cm s−1 with a counterflow in the lower layer of 2.8–3.1 cm s−1 towards the coast. 相似文献
15.
This paper discusses the variability of surface currents around Sekisei Lagoon using a nested grid ocean circulation model.
We developed a triple-nested grid system that consists of a coarse-resolution (1/60° or ∼1.85 km) model off Taiwan, an intermediate-resolution
(1/300° or ∼370 m) model around the Yaeyama Islands, and a fine-resolution (1/900° or ∼123 m) model of Sekisei Lagoon. The
nested grid system was forced by wind and heat flux calculated from six-hourly atmospheric reanalysis data and integrated
over the period from May to July 2003. The coarse-resolution model was driven by lateral boundary conditions calculated from
daily ocean reanalysis data to include realistic variation of the Kuroshio and mesoscale eddies with spatial scales of ∼500–700 km
in the open ocean. The tidal forcing was included in the intermediate-resolution model by interpolating sea level data obtained
from a data-assimilative tidal model. The results were then used to drive the fine-resolution model to simulate the surface
water circulation around Sekisei lagoon. Model results show that (1) currents inside the lagoon are mainly driven by tide
and wind; (2) there exists a strong southwestward current along the bottom slope in the southeast portion of the lagoon; the
current is mainly driven by remote mesoscale eddies and at times intensified by the local wind; (3) the flow relaxation scheme
is effective in reducing biases along the open boundaries. The simulated currents were used to examine the retention and dispersion
of passive particles in the surface layer. Results show that the surface dispersion in the strong open ocean current region
is significantly higher than that inside the lagoon. 相似文献
16.
Interaction of waves,currents and tides,and wave-energy impact on the beach area of Sylt Island 总被引:3,自引:1,他引:2
Erosion due to waves is an important and actual problem for most coastal areas of the North Sea. The objective of this study
was to estimate the impact of wave action on the coastline of Sylt Island. From a 2-year time series (November 1999 to October
2001) of hydrological and wave parameters generated with a coupled wave–current modelling system, a period comprising storm
‘Anatol’ (3–4 December 1999) is used to investigate the effects of waves on currents and water levels and the input of wave
energy into the coastline. The wave-induced stress causes an increase of the current velocity of 1 m/s over sand and an additional
drift along the coast of about 20 cm/s. This produces a water level increase of more than 20 cm in parts of the tidal basin.
The model system also calculates the wave energy input into the coastline. Scenario runs for December 1999 with a water level
increase of 50 cm and wind velocity increased by 10% show that the input of the wave energy into the west coast of Sylt Island
increases by 30% compared to present conditions. With regard to the forecasted near-future (Woth et al., Ocean Dyn 56:3–15,
2006) increase of strong storm surges, the scenario results indicate an increased risk of coastal erosion in the surf zone of
Sylt Island. 相似文献
17.
Paola Morasca Marco Massa Enrica Laprocina Kevin Mayeda Scott Phillips Luca Malagnini Daniele Spallarossa Giovanni Costa Paolo Augliera 《Journal of Seismology》2010,14(4):727-738
A merged, high-quality waveform dataset from different seismic networks has been used to improve our understanding of lateral
seismic attenuation for Northern Italy. In a previous study on the same region, Morasca et al. (Bull Seismol Soc Am 98:1936–1946,
2008) were able to resolve only a small area due to limited data coverage. For this reason, the interpretation of the attenuation
anomalies was difficult given the complexity of the region and the poor resolution of the available data. In order to better
understand the lateral changes in the crustal structure and thickness of this region, we selected 770 earthquakes recorded
by 54 stations for a total of almost 16,000 waveforms derived from seismic networks operating totally or partially in Northern
Italy. Direct S-wave and coda attenuation images were obtained using an amplitude ratio technique that eliminates source terms
from the formulation. Both direct and early-coda amplitudes are used as input for the inversions, and the results are compared.
Results were obtained for various frequency bands ranging between 0.3 and 25.0 Hz and in all cases show significant improvement
with respect to the previous study since the resolved area has been extended and more crossing paths have been used to image
smaller scale anomalies. Quality-factor estimates are consistent with the regional tectonic structure exhibiting a general
trend of low attenuation under the Po Plain basin and higher values for the Western Alps and Northern Apennines. The interpretation
of the results for the Eastern Alps is not simple, possibly because our resolution for this area is still not adequate to
resolve small-scale structures. 相似文献
18.
Irina G. Kudintseva Alexander P. Nickolaenko Masashi Hayakawa 《Surveys in Geophysics》2010,31(4):427-448
We describe the space–time distribution of the pulsed electric field in the middle atmosphere above a positive Γ-shaped lightning
stroke. The channel of such a discharge contains a vertical and a horizontal section. The current wave moves initially vertically
and then turns horizontally so that radiation appears from the vertical electric dipole followed by that from the horizontal
dipole. Combined with reflection from the perfectly conducting ground, the source provides three subsequent pulses in the
atmosphere, with the lag being determined by the finite velocity of the current wave in the Γ-shaped stroke. The pulses are
reproduced by reflections from the air-ground and the air-ionosphere interfaces and the waveform resembles the M-component,
which is often noted in the negative strokes (e.g. Yashunin et al., J Geophys Res 112:D10109, 2007). The non-stationary fine structure appears in the spatial distribution of electric field, which persists for 2 ms or even
more and exceeds the runaway electron threshold. Estimates support the idea of free electron bunching in the mesosphere by
the pulsed electric field. Focusing may occur about 10 km away from the point of electron- field interaction; it is delayed
by a few ms from the moment of interaction. The data presented might be helpful in realistic modeling of the red sprite formation. 相似文献
19.
Decadal evolution of a degassing magma reservoir unravelled from fire fountains produced at Etna volcano (Italy) between 1989 and 2001 总被引:1,自引:1,他引:0
Between 1989 and 2001, five eruptions at Etna displayed a regular alternation between repose periods and episodes rich in
gas, termed quasi-fire fountains and consisting of a series of Strombolian explosions sometimes leading to a fire fountain.
This behaviour results from the coalescence of a foam layer trapped at the top of the reservoir which was periodically rebuilt
prior to each episode (Vergniolle and Jaupart, J Geophys Res 95:2793–2809, 1990). Visual observations of fire fountains are combined with the foam dynamics to estimate the five degassing parameters characteristic
of the degassing reservoir, i.e. the number of bubbles, gas volume fraction, bubble diameter, reservoir thickness and reservoir
volume. The study of decadal cycles of eruptive patterns (Allard et al., Earth Sci Rev 78:85–114, 2006) suggests that the first eruption with fire fountains occurred in 1995 while the last one happened in 2001. The number of
bubbles and the gas volume fraction increase smoothly from the beginning of the cycle (1995) to its end (2001). The increasing
number of bubbles per cubic metre, from 0.61–20×105 to 0.1–3.4×109, results from cooling of the magma within the reservoir. The simultaneously decreasing bubble diameter, from 0.67–0.43 to
0.30–0.19 mm, is related to the decreasing amount of dissolved volatiles. Meanwhile, the thickness and the volume of the degassing
reservoir diminish, from values typical of the magma reservoir to values characteristic of a very thin bubbly layer, marking
the quasi-exhaustion of volatiles. The magma reservoir has a slender vertical shape, with a maximum thickness of 3,300–8,200 m
and a radius of 240 m (Vergniolle 2008), making its detection from seismic studies difficult. Its volume, at most 0.58–1.4 km3, is in agreement with geochemical studies (0.5 km3) (Le Cloarec and Pennisi, J Volcanol Geotherm Res 108:141–155, 2001). The time evolution of both the total gas volume expelled per eruption, and the inter-eruptive gas flux results from the
competition between the increasing number of bubbles and the decreasing bubble diameter. The smooth temporal evolution of
the five degassing parameters also points towards bubbles being produced by a self-induced mechanism within the magma reservoir
rather than by a magmatic reinjection prior to each eruption. The decadal cycles are therefore initiated by a magmatic reinjection,
in agreement with a typical return time of 14–80 years (Albarède 1993). Hence, the 1995 eruption results from a fresh magma being newly emplaced while the magma from the following eruptions is
progressively depleted in volatiles species until reaching a state of quasi-exhaustion in 2001. A magmatic reinjection of
0.13–0.6 km3 every few decades is sufficient to explain the expelled gas volume, including SO2. A scenario is also proposed for the alternation between gas-rich summit eruptions and gas-poor flank eruptions which are
observed during decadal cycles. The scenario proposed for Etna could also be at work at Piton de la Fournaise and Erta ’Ale
volcanoes. 相似文献
20.
Water levels and flows in the Singapore coastal waters are driven by the complex interactions of the Indian and Pacific Ocean
tides, seasonal monsoon-driven contributions and also forced by local winds. The Singapore Regional Model was developed to
simulate hydrodynamics in the Strait of Singapore which produces representative sea level variation in this region. However,
resolution and alignment of the grid system of the model with respect to depth contours in some of its subregions, i.e., the
Johor Estuary area require further improvement. For this, the grid system of the model was modified and compared the simulated
results with field measurements. The computed flow velocities agreed better with field observations when the grid resolution
was increased. However, improving the alignment of the grid with the channel boundary (with a much lower increase in grid
resolution) provided a substantially larger improvement of the model performance. The grid modification greatly influenced
the computed salinity in the estuary, while water levels are slightly affected. Further analysis of model results showed a
pronounced ebb tidal asymmetry generated by the O1–K1–M2 tidal constituents in the estuary. 相似文献