ISSN:0148-0227    

DOI:10.1029/92JB00365

年代:1992

数据来源: WILEY

摘要:

Fast and slow spreading ridges have radically different morphologic and gravimetric characteristics. In this study, altimeter measurements from the Geosat Exact Repeat Mission (Geosat ERM) are used to investigate spreading rate dependence of the ridge axis gravity field. Gravity roughness provides an estimate of the amplitude of the gravity anomaly and is robust to small errors in the location of the ridge axis. We compute gravity roughness as a weighted root mean square (RMS) of the vertical deflection at 438 ridge crossings on the mid‐ocean ridge system. Ridge axis gravity anomalies show a decrease in amplitude with increasing spreading rate up to an intermediate rate of ∼60–80 mm/yr and almost no change at higher rates; overall the roughness decreases by a factor of 10 between the lowest and highest rates. In addition to the amplitude decrease, the range of roughness values observed at a given spreading rate shows a similar order of magnitude decrease with transition between 60 and 80 mm/yr. The transition of ridge axis gravity is most apparent at three relatively unexplored locations on the Southeast Indian Ridge and the Pacific‐Antarctic Rise; on these intermediate rate ridges the transition occurs abruptly across transform

ISSN:0148-0227    

DOI:10.1029/91JB02465

年代:1992

数据来源: WILEY

摘要:

In August 1990, the U.S. Navy declassified all Geodetic Mission (GM) radar altimeter data acquired by the Geosat satellite over oceanic regions south of 60°S. We have used these GM data in conjunction with the unclassified, lower‐resolution Geosat Exact Repeat Mission (ERM) altimeter data to construct high‐resolution gravity fields on a 5‐km grid covering the annular region of the southern ocean, which lies between 60°S and 72°S and encircles Antarctica. During the GM a complete mapping of the marine geoid (between 72° and 72°N) was accomplished. The GM produced more densely spaced ground tracks (typically 2 or 3 km at 60°S) than those of either the ERM or Seasat. Consequently, we were able to use the GM data to map the marine gravity field at a higher resolution than was previously possible using satellite altimeter data. This paper describes the techniques we used to derive these gravity fields and image them. These techniques involve (1) computing along‐track sea surface height slopes, (2) gridding of these ascending and descending slopes, (3) converting the slopes to conventional deflections of the vertical, (4) transforming the deflections to gravity anomalies in the frequency domain, and (5) imaging. The resulting images of the marine gravity field reveal much that is new about the seafloor and the tectonic fabric of the southern ocean: a region which includes large expanses of seafloor that have never been surv

ISSN:0148-0227    

DOI:10.1029/91JB02797

年代:1992

数据来源: WILEY

摘要:

On the basis of the analysis of Global Positioning System (GPS) data collected at the U.S. Geological Survey Kennedy Ranch House alinement array near Parkfield, California, in November 1990 and February 1991, we assess the temporal resolution of crustal deformation for GPS receiver systems operating as strain meters and provide guidelines for achieving millimeter‐level precision with short‐range, kinematic‐type GPS measurements. Site occupation of about one‐half hour is sufficient for day‐to‐day repeatability of submillimeter precision in relative position during good satellite geometry. Signal multipath effects which dominate dispersive baseline errors are essentially averaged out and need not be modeled. However, as occupation times decrease, baseline solutions are increasingly sensitive to multipath effects which together with nondispersive receiver measurement noise, are of millimeter to centimeter magnitude in epoch‐by‐epoch solutions. On repeatedly monitored baselines we can account for both of these noise sources separately. The mostly random receiver noise is effectively suppressed by low‐pass filters with corner frequencies of about 20 mHz. The multipath noise is filtered using the fact that it essentially repeats from day to day. Residual errors in filtered 1‐s epoch‐by‐epoch solutions are below 2 mm during periods of good satellite geometry. Hence for a continuously monitored baseline, actual deformation in excess of this threshold can be detected with a time lag comparable to the group delay of the low‐pass filter, i.e., typically within a minute. In terms of strain along a typical short baseline of 1 km sampled once per second we can detect 1–2 μs in l min with multipath modeling. Without multipath modeling, it takes 10 min to resolve the same magnitude of deformation. In 30 min we can detect 0.1 μs independent of multipath modeling. These results have important implications for kinematic‐type GPS surveys. For millimeter‐level precision without impractical multipath modeling it is necessary to occupy a site during good satellite geometry for a period longer than that of the multipath signature. Our experience in both benign and difficult multipath environments indicates that approximately 10 min are sufficient. The measurements at the Kennedy Ranch House alinement array suggest that this segment of the San Andreas fault appears to be virtually locked at the surface during the 3‐m

ISSN:0148-0227    

DOI:10.1029/91JB02997

年代:1992

数据来源: WILEY

摘要:

This paper describes an ambiguity function technique by which phase data from the Global Positioning System (GPS) maybe used to obtain survey positions for monuments occupied using a suitable GPS receiver. The principal benefit of the technique is the speed with which it may be used and its immunity to cycle slips or other discontinuities in the phase data that commonly occur. The technique uses phase measurements from satellites at both L1and L2frequencies from various directions in the sky. When sufficient measurements are available, the signals will destructively interfere at all but the correct location. Depending on the satellite geometry, the number of satellites in view, and the a priori knowledge of the station's position, a sufficient number of measurements may be obtained from a single monument occupation. The data collected over an occupation lasting only a few minutes maybe averaged to a single value at a particular epoch. Equivalently, the data collected at only a single epoch may also be used. The ability of the technique to work with data from a single epoch allows the integer ambiguities on moving platforms to also be determined. This will greatly enhance the practical operation of kinematic GPS for a variety of remote sensing applications. A computer program employing this ambiguity function technique has been written and used to process static as well as kinematic data. The results indicate horizontal precisions and accuracies of 1 cm or better and vertical precisions and accuracies of 1 to 3 cm. The data processing requires no data editing, is easy to use, and can be operated without any specialized knowledge of kinematic GPS techniques. The correction for the effects of the ionosphere are also demonstrated at distances up to 250 km. The use of the ambiguity function for geophysical monitoring and kinematic remote sensing applications appears limited in distance only by the accuracy of the GPS orbits. The utility of the ambiguity function for rapid static surveying and on‐the‐fly bias fixing out to distances of several hundred kilometers appears practi

ISSN:0148-0227    

DOI:10.1029/91JB02845

年代:1992

数据来源: WILEY

摘要:

A global data base consisting of radiocarbon‐controlled relative sea level (RSL) histories from 392 different geographic locations has been compiled for use in analyses of the glacial isostatic adjustment process. These data may be employed to constrain both the spatiotemporal characteristics of the last deglaciation event of the current ice age and the rheological structure of the planetary interior. In a previous paper in this series (Tushingham and Peltier, 1991), we employed the RSL data in this set from sites that were once ice covered to refine the apriori model of deglaciation that had been delivered by past research in the area of Pleistocene geomorphology (Peltier and Andrews, 1976; Wu and Peltier, 1983) to produce a new global model of this event that we have called ICE‐3G. In the present paper, following a description of the full data base and of the methods used to compile it, we employ data from the sites that were not ice covered (200 time series) to verify the plausibility of this refined model of deglaciation and to test the extent to which the totality of the observations constrain the radial viscoelastic structure of the planet. The analysis of solutions to the forward problem presented here reinforce previous inferences of a preference for a thick lithosphere by RSL data from sites along the U.S. east coast and a preference by data from most near‐field sites for an increase in viscosity across the 670‐km discontinuity in the mantle by a factor of approximately 2, when the viscosity of the upper mantle is fixed to the value of 1021Pa s originally deduced by Haskel (1935) on the basis of his analysis of the data from Fenno

ISSN:0148-0227    

DOI:10.1029/91JB02176

年代:1992

数据来源: WILEY

摘要:

We use leveling and trilateration data collected on Kilauea volcano to constrain the location of deformation sources caused by magma accumulation, intrusion, and eruption. For the 13 inflationary epochs examined, combinations of an expanding point source and one or two opening rectangular dislocations mimic inflation of the summit reservoir and formation of dike(s), respectively. The combined model adequately accounts for the deformation data and is consistent with the seismicity observed during each epoch. For 10 deflationary epochs, however, the data require only a contracting point source. Confidence in these results is gained by noting that locations of the sources of both inflation and deflation are coincident, within the observed uncertainties of the data, the function of network geometry, and the inversion procedure. It appears, therefore, that magma accumulation at Kilauea volcano may be characterized by the growth of dikes during inflation of the summit reservoir. Drainage of the reservoir, on the other hand, is not accompanied by significant closure of dikes. In contrast to previous studies (e.g., Fiske and Kinoshita, 1969; Dvorak et al., 1983) that do not include the dislocation (or dike growth) component of summit magma accumulation and concluded that the source of inflation migrates over a 5 km2area, we find that a single magmatic reservoir source accounts for data collected during all inflationary and deflationary epochs, results, which compare favorably with those obtained from the point ellipsoid model, can be used to estimate the distribution of stresses within the volcano in the near field of the source.

ISSN:0148-0227    

DOI:10.1029/91JB02842

年代:1992

数据来源: WILEY

摘要:

We present an approximate analytical solution for the effects of topography on near‐surface gravity‐induced stresses in symmetric anisotropic ridges and valleys. The solution can be used for laterally constrained ridges and valleys consisting of isotropic, transversely isotropic, or orthotropic rock with horizontal or vertical layers. A parametric study on the effect of anisotropy type and degree of rock anisotropy on gravity‐induced stresses is presented. The magnitude of the predicted stresses is of the order of the characteristic stress ρgH, whereHis the height of the ridge or depth of the valley. The approximate solution is limited to ridges and valleys with small slopes not exceedi

ISSN:0148-0227    

DOI:10.1029/91JB02960

年代:1992

数据来源: WILEY

摘要:

We present a new tectonic fabric map of the northern and central North Atlantic Ocean floor from 10°S to 66°N based on combined Geosat and Seasat altimetry data. Short wavelength vertical gravity along‐track was derived by Hilbert transforming and high‐pass filtering Geosat and Seasat deflection of the vertical profiles. The along‐track gravity data were used in conjunction with magnetic anomaly data to locate a large number of fracture zones as well as extinct spreading ridges and V‐shaped structures. We traced many fracture zones more continuously and closer to adjacent continental margins than was previously possible. We also mapped a number of unnamed fracture zones and fracture zone segments, many of which were previously uncharted. We have evaluated the agreement between the mapped fracture zones and synthetic flow lines as predicted from recent plate models for the opening of the North, central North and South Atlantic. Most fracture zones in this area correlate quite well with synthetic flow lines, supporting the proposition that Geosat/Seasat altimetry data serve as a useful tool to map fracture zones, where insufficient bathymetric data are available. Many fracture zones, including some small‐offset fracture zones in the central North Atlantic, have been stable and follow tectonic flow lines for tens of millions of years. However, many other fracture zones do not reflect a ″railroad track″ pattern of plate motions. The geometry of many small‐ and medium‐offset fracture zones indicates that the ridge axis discontinuities that produce them can migrate along the ridge, causing off‐axis traces that deviate from tectonic flow lines. Velocity‐space diagrams constructed using a recent relative and absolute present day plate motion model indicate that some low‐angle V‐shaped troughs and ridges are likely formed by migration of the plate boundary relative to a mesospheric reference frame. Other low‐angle V‐shaped structures reflect slow migrations/oscillations of small‐offset ridge axis discontinuities that are distinct from both tectonic flow lines and V‐shapes reflecting “absolute” plate motions. High‐angle V‐shaped structures are the result of fast migrating rid

ISSN:0148-0227    

DOI:10.1029/91JB02605

年代:1992

数据来源: WILEY

摘要:

Eight magnetotelluric (MT) soundings were performed as a continuation of an earlier audiomagnetotelluric (AMT) survey conducted in the northeastern border of the Paraná basin, a large intracratonic basin located in central eastern South America and constituted by Silurian to Jurassic sedimentary rocks with Lower Cretaceous sill‐type magmatic intrusions and overlying volcanics. Two of the MT stations were carried out near two deep petroleum wells. The remaining MT soundings were done on a profile traversing two important gravity features: a positive anomaly near the border of the states of São Paulo and Minas Gerais and a strong (trending NW‐SE) linear gradient. Major findings of an integrated interpretation of the MT survey and of available gravity data are as follows: (1) inhomogeneities and/or strong magnetization of the superficial volcanics and diabase intrusions in the Paleozoic sedimentary rocks appearing to distort the MT results; and (2) identification of important structural discontinuities, including a possible different crustal structure beneath the Paraná basin compared to the region on the north, a thickening of the crust toward the NE, and the probable existence of a trough (graben?) within the basin, characterized by a thick accumulation of sediments and b

ISSN:0148-0227    

DOI:10.1029/91JB02712

年代:1992

数据来源: WILEY