Gravity and geoid anomalies of the Philippine Sea: Evidence on the depth of compensation for the negative residual water depth anomaly

A negative free-air gravity anomaly which occurs in the central part of the Philippine Sea was examined to determine the distribution and nature of possible regional mass excesses or deficiencies. Geoid anomalies from GEOS-3 observation were positive. A negative residual geoid anomaly consistent with the area of negative free-air gravity anomalies were found. Theoretical gravity-topography and geoid-topography admittance functions indicated that high density mantle at about 60 km dept could account for the magnitudes of the gravity and residual geoid anomaly and the 1 km residual water depth anomaly in the Philippine Sea. The negative residual depth anomaly may be compensated for by excess density in the uppermost mantle, but the residual geoid and regional free-air gravity anomalies and a slow surface wave velocity structure might result from low-density warm upper mantle material lying beneath the zone of high-density uppermost mantle. From a horizontal disk approximation, the depth of the low-density warm mantle was estimated to be on the order of 200 km

Processing MAGSAT data for comparison with geoid anomalies

A digital data library of MAGSAT data consisting of 1,615,636 measurements from the quiet data set, is geographically sorted, and allows rapid analysis and processing of all the quiet magnetic data about any selected location. Because this library of MAGSAT data is compatible with existing gravity and geoid data library processing and display system software, correlations between MAGSAT, surface gravity, GEOS-3 radar altimeter geoid and bathymetric data sets can be conveniently detected and analyzed. Polynomial trends from each half-orbit were removed as an effective way of estimating and removing ring current effects following estimation of the core field contribution. It was found that a third order polynomial is the lowest polynomial order that appears to provide the best consistency of residual anomalies between coincident orbits

Gravity and crustal structure

Lunar gravitational properties were analyzed along with the development of flat moon and curved moon computer models. Gravity anomalies and mascons were given particular attention. Geophysical and geological considerations were included, and comparisons were made between the gravitional fields of the Earth, Mars, and the Moon

Catalogue of geoidal variations for simple seafloor topographic features

A catalogue is presented of theoretical geoidal variations for three types of structural features common to the earth's surface: seamounts, submarine ridges, and submarine trenches. These structures were simulated by simple geometric shapes modeled in three-dimensions. A computer program calculated the potential and gravitational variations over the models. Profile plots of geoidal variations and free-air gravity anomalies are presented over cross-sections of the structures. A ready reference information set is provided for comparison with satellite altimeter data for ocean areas

MAGSAT correlations with geoid anomalies

The MAGSAT data of the Gulf of Mexico were analyzed to define better the possible relation of the negative MAGSAT anomaly there to the negative residual geoid anomaly in the western Gulf of Mexico. The estimated magnetic crystal anomaly pattern has a magnetic low in the region of the residual geoid low, but the shape of the anomalies are different. Since the shape and location of the negative magnetic anomaly are variable depending upon the particular polynomial and curve orders used, the degree of correspondance between the residual geoid and MAGSAT lithosphere anomalies was not established definitively

Gravity data obtained during Chain cruise 73

This report presents the results of gravity measurements obtained aboard the Research Vessel CHAIN of the Woods Hole Oceanographic Institution during cruise 7 3, 6 to 26 September 1967. Information on bathymetry and other studies conducted during CHAIN cruise 73 may be found in W.H.O.I. Reference No. 68-50 (Knott, et al., 1968). Gravity measurements were made simultaneously with the Massachusetts Institute of Technology (MIT) vibrating string gravity meter and with LaCoste and Romberg sea gravity meter number S-13. The data presented is a selective composite of information from both meters . The base reference station used for the measurements is at the W. H. 0. I. dock in Woods Hole, Massachusetts. Its gravity value was taken to be 980327. 0 mgal, however, recent ties of this station to national gravity network stations (Anonymous, 1965) in the Boston, Massachusetts area indicate that this reading is high by about 1. 4 mgal. Thermal and electrical difficulties with the MIT meter made port-to-port drift prediction uncertain (Bowin, Wing and Aldrich, in press). Drift rate for this meter was determined on the basis of its continuous averaged differences with the LaCoste and Romberg meter. This method showed the MIT meter drifting with respect to the LaCoste and Romberg meter by+ 1. 611 mgal per day for the cruise period. Drift of the LaCoste and Romberg gravity meter during CHAIN 73 was - 0.026 mgal/day from in-port measurements at beginning and end of cruise. This last drift value is anomalous in sign and magnitude with respect to the meter's over-all drift rate , and was not applied. The accuracy of the sea measurements is estimated to be about ±10 mgal (Bowin, Wing, and Aldrich, in press). Free -air anomaly values were calculated using the 1930 International Gravity Formula. Water depths are in meters corrected according to the tables of Matthews ( 1939). Bouguer anomaly values were calculated using an infinite slab correction and an assumed crustal density of 2. 67 gm/cm3 . A density of 1. 03 gm/cm3 was assumed for sea water.The National Science Foundation Grant GA-9?6 and submitted to the Office of Naval Research under Contract Nonr-4029(00)~ NR 260-101

Analysis of satellite gravity and bathymetry data over Ninety-East Ridge: Variation in the compensation mechanism and implication for emplacement process

International audienceWe investigate the mode of compensation, emplacement history and deep density structure of the Ninety-East Ridge (Indian Ocean) using spectral analyses and forward modeling of satellite gravity and bathymetry data. We find that the northern (0–10°N) and the southern (20–30°S) parts of the ridge are flexurally compensated with an effective elastic thickness >15 km, whereas the central part (0–20°S) is locally compensated. Furthermore, we find that for a part of central block (10–20°S, over Osborn Knoll) the compensation depth is unreasonably very high (30–40 km). Therefore we favor a model with subsurface loading and interpret this to be due to underplating of mafic material at the base of the crust, a hypothesis that is supported by seismic results and direct modeling of gravity data along some profiles. These results suggest that the northern and southern parts of Ninety-East Ridge were emplaced off to a ridge axis compared to the central one, which might have been emplaced on or near a spreading center. Locally compensated large topography, thick underplated crust in the central part (near Osborn Knoll), might result from an interaction of a hot spot with the extinct Wharton spreading ridge

Development of a Synthetic Earth Gravity Model by 3D mass optimisation based on forward modelling

Several previous Synthetic Earth Gravity Model (SEGM) simulations are based on existing information about the Earth’s internal mass distribution. However, currently available information is insufficient to model the Earth’s anomalous gravity field on a global scale. The low-frequency information is missing when modelling only topography, bathymetry and crust (including the Mohorovičić discontinuity), but the inclusion of information on the mantle and core does not seem to significantly improve this situation. This paper presents a method to determine a more realistic SEGM by considering simulated 3D mass distributions within the upper mantle as a proxy for all unmodelled masses within the Earth.The aim is to improve an initial SEGM based on forward gravity modelling of the topography, bathymetry and crust such that the missing low-frequency information is now included. The simulated 3D mass distribution has been derived through an interactive and iterative mass model optimisation algorithm, which minimises geoid height differences with respect to a degree-360 spherical harmonic expansion of the EGM2008 global external gravity field model. We present the developed optimisation algorithm by applying it to the development of a global SEGM that gives a reasonably close fit to EGM2008, and certainly closer than a SEGM based only on the topography, bathymetry and crust

The Mw = 6.3, November 21, 2004, Les Saintes earthquake (Guadeloupe): Tectonic setting, slip model and static stress changes,

International audienceOn November 21, 2004, a magnitude 6.3 earthquake occurred offshore, 10 km south of Les Saintes archipelago in Guadeloupe (French West Indies). There were more than 30000 aftershocks recorded in the following two years, most of them at shallow depth near the islands of the archipelago. The main shock and its main aftershock of February 14, 2005 (Mw = 5.8) ruptured a NE-dipping normal fault (Roseau fault), mapped and identified as active from high-resolution bathymetric data a few years before. This fault belongs to an arc-parallel en echelon fault system that follows the inner edge of the northern part of the Lesser Antilles arc, accommodating the sinistral component of oblique convergence between the North American and Caribbean plates. The distribution of aftershocks and damage (destruction and landslides) are consistent with the main fault plane location and attitude. The slip model of the main shock, obtained by inverting jointly global broadband and local strong motion records, is characterized by two main slip zones located 5 to 10 km to the SE and NW of the hypocenter. The main shock is shown to have increased the Coulomb stress at the tips of the ruptured plane by more than 4 bars where most of the aftershocks occurred, implying that failures on fault system were mainly promoted by static stress changes. The earthquake also had an effect on volcanic activity since the Boiling Lake in Dominica drained twice, probably as a result of the extensional strain induced by the earthquake and its main aftershock

Potentiality in Aristotle's psychology and ethics

The distinction between potentiality and actuality in Aristotle has its origin in Platonic ethics. In his psychological and ethical works Aristotle’s notion of potentiality is embedded in a causal framework that is characteristic of life in general. A key theme is the distinction of various meanings of ‘to know’. In his early work the possession of knowledge is distinguished from its use. In De anima Aristotle adds the potentiality for acquiring knowledge as characteristic of the genus human being. He argues that the stages of actualization of knowledge are instances of a more comprehensive biological and ethical development. Life is the fulfillment of soul as formal, efficient and final cause, with the potentiality of body as material cause. The unity of body and soul is derived from the causal nexus of potentiality and actuality, like a power and the instrument in which it resides. In such cases potentiality is complex and depends on numerous conditions. Failure of full realization may occur when any of the necessary conditions of the development and realization of the fulfillment of human life are lacking, whether in the environment (e.g. climate), the body (illness, drunkenness), or the soul (natural virtue, firm character, attention).Political Philosophy and Ethic