Analysis of the magnetic anomaly field of the volcanic district of the Bay of Naples, Italy

We here present and discuss the results of the analysis and qualitative interpretation of two magnetic surveys performed in the Bay of Naples in 1998 and 2000. A map of the Bay of Naples based on the data acquired during these surveys has already been published by the Italian CNR-IAMC Research Institute. We re-processed the same data to produce maps of the pole reduced, analytic signal and horizontal derivative data and correlated them with the bathymetry and the gravimetric data of the area. The analysis shows strong anomalies in the NW and NE volcanic areas of the Bay of Naples, while the central area seems magnetically quiet. In the Phlegrean area the maps clearly show the southern rim of the Phlegrean caldera and demonstrate that while the Magnaghi Canyon is correlated to gravimetric highs and magnetic structures, and can therefore be interpreted as an active lineament, most of Dohrn Canyon is not characterized by volcanic activity and does not correlate to any gravimetric or magnetic structures. An important round-shaped magnetic anomaly is for the first time identified in the central slope of the gulf between the two canyons, probably correlated to a large buried volcanic edifice. In the Vesuvian area some intense circular anomalies, aligned in the NNW–SSE direction, are localized in the Torre del Greco and Torre Annunziata offshore, related to the submerged part of Vesuvius and possibly connected to buried vents.     

Dynamic Response of Fluid-Single Leg Gravity Platform-Soil Interaction System

- An approximate method is presented to investigate the earthquake response of the fluid-single leg (shortened for S. L.) gravity platform-soil interaction system. By assuming a suitable form of the velocity potential of the radiation waves and by using the motion equation and the boundary conditions, the unknown coefficients can be obtained. Thereafter the function of frequency for the interaction system may also be obtained. In this paper, the difference of the system dynamic response between rigid foundation is analyzed and the influences of the various foundation geometric dimension and the various water-depth on the hydrodynamic loading and dynamic response of the system is illustrated.     

Hilbert transform and its application to modulation of capillary gravity waves

INTRODUCFIONlt is well known that the stability and interaction of nonlinear waves lead to modulations ofwavenumber and frequency, as well as arnplitude, with the frequency dePendent on bothwavenumber and arnplitude (Witham, l974; Philips, l98l ). These medulations are not justtheoretical curiossities but have important conSequences in oceanography and meteorology, aswell as naval architecture and ocean engineering (honguer-Higgins, l980).By means of Hilbert transfOrm technique, Melvill…     

Gravity anomalies and deep structure of the Ninetyeast Ridge north of the equator,eastern Indian Ocean — a hot spot trace model

The Ninetyeast Ridge north of the equator in the eastern Indian Ocean is actively deforming as evidenced by seismicity and its eastward subduction below the Andaman Trench. Basement of the ridge is elevated nearly 2 km with respect to the Bengal Fan; seismic surveys demonstrate continuity of the ridge beneath sediment for 700 km north of 10° N where the ridge plunges below the Fan sediment. The ridge is characterised by a free-air gravity high of 50 mgal amplitude and 350 km wavelength, and along-strike continuity of 1500 km in a north-south direction, closely fringing (locally, even abutting) the Andaman arc-trench bipolar gravity field. Regression analysis between gravity and bathymetry indicates that the ridge gravity field cannot be explained solely by its elevation. The ridge gravity field becomes gradually subdued northwards where overlying Bengal Fan sediments have a smaller density contrast with the ridge material. Our gravity interpretation, partly constrained by seismic data, infers that the ridge overlies significant crustal mass anomalies consistent with the hot spot model for the ridge. The anomalous mass is less dense by about 0.27 g cm^–3 than the surrounding oceanic upper mantle, and acts as a cushion for isostatic compensation of the ridge at the base of the crust. This cushion is up to 8 km thick and 400–600 km wide. Additional complexities are created by partial subduction of the ridge below the Andaman Trench that locally modifies the arc-trench gravity field.     

Simulation and Analysis of a Gravity Cyh''''nder''''s Stability in Short-Crested Sea

Generally, the reliability of ami -sliding and anti-overturning stability of an isolated gravity cylinder in a certain working period can be evaluated only when the statistical properties of short term stability are given first. The authors used numerical method to simulate the stability state function of a cylinder in short-crested sea, and further to get the probabilitical characteristics of the structure's stability by time domain analysis. The external loads appeared in the state functions include horizontal wave force, lift force and the respective moments, and the loads are correlated by co- spectrum. The numerical method presented in this paper can be used not only to solve short term reliability problem directly, but to calculate and analyse the long term reliability problem as well. For circular cylinders, an example of simulation and analysis is displayed in this paper.     

Water-level fluctuations, emersion regimes, and variations of echinoid populations on a Caribbean reef flat

This paper examines fluctuations in water level over a Caribbean reef flat at Punta Galeta, Panamá. In an analysis of approximately ten years of records, the mean diurnal range of the tides was 24·5 cm and varied <2 cm from year to year. Daily mean water levels varied erratically over a range of approximately 30 cm. Monthly mean water levels fluctuated seasonally over a range of about 10 cm and were consistently higher than those at the regional tidal reference station at Cristóbal. On days with more wave action, water levels at Galeta increased relative to those at Cristóbal, suggesting that waves were ‘pumping’ water onto the reef flat. The monthly mean water levels at the two sites were not correlated, indicating that tidal data from conventional stations in deeper water cannot be extrapolated to reef flats, except as estimates of minimum potential water levels.Most of the reef flat was within 6 cm vertical span just below mean lower low water. The highest elevations within this range were exposed above water level for an average of 918 hours per year, as compared with 144 hours per year at the lowest elevation. Most exposures lasted less than 10 hours, with a modal duration of 3–5 hours; however, exposures longer than 12 hours occurred at nearly all elevations in all years. Exposures of the reef flat were most frequent between February and June and between August and November, a pattern apparently caused by a combination of seasonal oscillations of regional sea levels, the annual pattern of the solstitial tides, and waves generated by seasonal trade winds.The fluctuations in water levels apparently affected the abundances of some species of sea urchins on the reef flat. Populations of Lytechinus variegatus and Diadema antillarum declined or disappeared from the reef flat during seasons of repeated subaerial exposures, but recolonized the habitat in periods of higher water levels. Although they are reported to suffer heavy mortality during emersion, Echinometra lucunter and E. viridis did not exhibit reductions in abundance that were synchronous with the seasonal exposures of the reef flat.     

Improved Arctic Ocean Mass Variability Inferred from Time-Variable Gravity with Constraints and Dual Leakage Correction

Abstract

The ocean mass variability inferred from Gravity Recovery and Climate Experiment (GRACE) satellites mission is challenged by the stripes and the leakage across land-ocean boundary. The recently released GRACE mascons solutions are advanced by applying constraints that remove efficiently the stripes and dual leakage correction that restores the coastal ocean mass variability. Here we quantitatively evaluate the improvement in the Arctic Ocean mass variability by GRACE mascons. To do so, we compare the combination of GRACE solutions (including the mascons solutions and traditional spherical harmonic coefficients (SHCs) solutions) and the steric estimates against the altimeter observations. Our results suggest that mascons solutions produce stronger correlations compared to SHCs solutions, especially along the coastal zone, indicating the importance of the dual leakage correction. Stronger correlation is produced by the mascons over a small basin in the interior of the Arctic Ocean, suggesting that mascons solutions deliver better ocean mass variability than the SHCs solutions. Since the comparisons are carried out over two sub-basins, we conclude that mascons are able to provide better regional ocean mass variability that may have implications for regional sea level budget, in particular over the coastal zone.     

利用重磁计算解释南海海盆中部地壳结构特征

以南海海盆中部3个海盆的交汇处为研究区，通过对重磁异常场的向上延拓计算及对重力异常的重反演拟合计算，并结合地质资料，进行综合分析和解释来揭示此区域的地壳结构特征。结果表明研究区地壳结构复杂，三种性质的地壳均有分布。各海山形成过程和岩石组成及地壳结构不尽相同，区内有年轻的大洋玄武岩海山，也有包含了大陆玄武岩的“双性”海山。综合各种地质、地球物理现象，认为南海海盆可能是多种机制联合作用的结果，西南、西北两海盆的形成时间要早于中央海盆。     

Effects of Meteorological Factors on Toxic Red Tide Events in Sabah, Malaysia

Abstract. Red tides of the toxic dinoflagellate Pyrodinium bahamense var. compressum have occurred almost annually since 1976 in the coastal waters of the west coast of Sabah, Malaysia. Field obSérvations have indicated that the timings of these outbreaks could be linked to meteorological forcings. All the red tides have occurred during either the northeast or southwest monsoon periods. Matching of red tide outbreaks with total monthly rainfall, monthly-averaged index of wind-induced surface water mixing, and monthly-averaged upwelling index for the period from 1975 to 1989 failed to show any definite pattern. However, more red tide events occurred in months with high mixing index values. The absence of definable patterns could be due to the use of monthly, rather than daily, values in the calculations.